HI all
After a problem with sticking disk brakes last year, and some very helpful responses from you guys for which I'm very grateful, I replaced my master cylinder on my 1970 Cuda (after having also reconditioned the disk calipers etc, and replaced the flexi hoses to the caliper). This has fixed my previous issue with sticking disk calipers.
After putting it all back together, I've bled the brakes up twice now (all 4 wheels, starting from furthest from the master cylinder as other posts advised) and the brakes seem to be fine with the engine off: the pedal goes about half way down, and is very firm. It doesn't firm up any further with pumping.
The problem comes when I start the engine and the booster kicks in. With my foot on the pedal, it immediately falls further - to about 3 quarters of the way down, and (keeping pressure on it with the engine running) it continues to fall slowly till it's very close to the floor within a further 10-15 seconds. (I don't think it finishes quite at the floor, but it's close).
I don't understand why applying the booster should have the effect of allowing the pedal to fall further. I know it makes it easier to push down the pedal, and mechanically applies the booster pushrod into the master cylinder, but not sure why it would allow it to go further down than pushing hard without the booster. Any ideas please?
And any thoughts on what I should do next to get the brake pedal to hold up when the engine and booster ar running? Am not sure what the underlying problem is - still air somewhere in the system? (It bleeds clear with no bubbles, and I bench bled the new master cyclinder as well). Or something with the booster? (although that seems to be working fine in that it has the effect it has!). Or something else? I'm stumped.
Many thanks in advance!
Cheers
Pete
Brake pedal sinking with engine
Moderator: Moderators
Brake pedal sinking with engine
1970 Plymouth Cuda 383.
Re: Brake pedal sinking with engine
the booster just gives you the equivelent double strength leg muscles
if your foot combined with with the boosters big boot cause the peddle to sink it sounds like air or a leak that you can only bring into the mix with foot and booster power..
keep in mind booster brakes have a different peddle ratio so the pressure you create with just your foot and no booster is quite a bit less than what you would create in the system on a manual set up
if it dropped on starting the booster is active when the peddle isn't pressed i.e the rod from peddle cluster into booster/mastercylinder is too long
but it doesn't do that by the sound of things it just drops further in use.... wonder if there is air in front still
i doubt you have a soft line, balooning, as it sounds like its all new
easy to get high pressure in the circuit for the rear... drum brakes take low line pressure to be fully ON, air in the rears and your peddle would be dropped to the floor. the system is smaller in total less places for air to hide and a striaght shot at the bleedre for getting it out.
disk calipers take massive pressure to be fully on due to the huge difference in size between a disk brake caliper piston and the tiny piston in a wheel cylinder out back.
if there was a leak there would be a drip or spray... you'd see it eventually
hence i think its air in the front and it effect is only noticable when you and the booster supply your considerbale thrust to the peddle
do keep in mind i'm often completely wrong and all ive done is apply abit of logic to the way i think your brakes work
it all based on the fact that my manual brakes were a pig to bleed and it was air in the front that hung around longer
10 minutes of stamping the peddle eventually shifted it, presume the fluid got hot and the air bubble expanded to the point where it reached an exit up to the master. i had got to the point where i was going to remove the calipers stick a block of wood in each and hang them so the highest point was the bleeder.. thankfully i didn't need to
other options
nothing wrong and you are witnessing the effect of warped backing or anti rattle shim on the disk pads (it will bed in) or the brand new seals on the pistons in your new calipers are pulling the pistons back half a mile when you take your foot off the peddle, due to stiffness in the rubber and beautiful un rusty pistons. will soften in use and the situation will improve.
i think if it was master cylinder problem you'd see it with engine running and without
Dave
if your foot combined with with the boosters big boot cause the peddle to sink it sounds like air or a leak that you can only bring into the mix with foot and booster power..
keep in mind booster brakes have a different peddle ratio so the pressure you create with just your foot and no booster is quite a bit less than what you would create in the system on a manual set up
if it dropped on starting the booster is active when the peddle isn't pressed i.e the rod from peddle cluster into booster/mastercylinder is too long
but it doesn't do that by the sound of things it just drops further in use.... wonder if there is air in front still
i doubt you have a soft line, balooning, as it sounds like its all new
easy to get high pressure in the circuit for the rear... drum brakes take low line pressure to be fully ON, air in the rears and your peddle would be dropped to the floor. the system is smaller in total less places for air to hide and a striaght shot at the bleedre for getting it out.
disk calipers take massive pressure to be fully on due to the huge difference in size between a disk brake caliper piston and the tiny piston in a wheel cylinder out back.
if there was a leak there would be a drip or spray... you'd see it eventually
hence i think its air in the front and it effect is only noticable when you and the booster supply your considerbale thrust to the peddle
do keep in mind i'm often completely wrong and all ive done is apply abit of logic to the way i think your brakes work
it all based on the fact that my manual brakes were a pig to bleed and it was air in the front that hung around longer
10 minutes of stamping the peddle eventually shifted it, presume the fluid got hot and the air bubble expanded to the point where it reached an exit up to the master. i had got to the point where i was going to remove the calipers stick a block of wood in each and hang them so the highest point was the bleeder.. thankfully i didn't need to
other options
nothing wrong and you are witnessing the effect of warped backing or anti rattle shim on the disk pads (it will bed in) or the brand new seals on the pistons in your new calipers are pulling the pistons back half a mile when you take your foot off the peddle, due to stiffness in the rubber and beautiful un rusty pistons. will soften in use and the situation will improve.
i think if it was master cylinder problem you'd see it with engine running and without
Dave
The Greater Knapweed near the Mugwort by the Buckthorn tree is dying
Re: Brake pedal sinking with engine
Thanks Dave, much appreciated, and very helpful (as usual!).
All your assumptions are right - new kit etc, no leaks visible anywhere. And to confirm: Pedal falls only when foot pressure is on the brake AND engine started, so don't believe it's the booster rod length. Also thanks for confirming that booster appears to be behaving normally.
Re air in the front part of the system: Can I just get you to expand a bit on "stamping on pedal for 10 mins". Is this with everything connected up normally, or when you're bleeding it? I assumed it's the former, ie that you're just trying to get the air to move up towards the bleed screws first by some rapid pumping, before then re-bleeding at the calipers?
One other thing: when I bench bled the new master cylinder, I only did it for a minute or so (maybe 5 or 6 pumps of the piston) until bubbles stopped. I'm wondering if I should have waited a bit longer and done it a number of times, allowing any remaining air to settle between goes. I'm wondering if a small amount of air could have been trapped in there. I've seen a couple of other older posts that suggest this as a potential cause.
Cheers again
Pete
All your assumptions are right - new kit etc, no leaks visible anywhere. And to confirm: Pedal falls only when foot pressure is on the brake AND engine started, so don't believe it's the booster rod length. Also thanks for confirming that booster appears to be behaving normally.
Re air in the front part of the system: Can I just get you to expand a bit on "stamping on pedal for 10 mins". Is this with everything connected up normally, or when you're bleeding it? I assumed it's the former, ie that you're just trying to get the air to move up towards the bleed screws first by some rapid pumping, before then re-bleeding at the calipers?
One other thing: when I bench bled the new master cylinder, I only did it for a minute or so (maybe 5 or 6 pumps of the piston) until bubbles stopped. I'm wondering if I should have waited a bit longer and done it a number of times, allowing any remaining air to settle between goes. I'm wondering if a small amount of air could have been trapped in there. I've seen a couple of other older posts that suggest this as a potential cause.
Cheers again
Pete
1970 Plymouth Cuda 383.
Re: Brake pedal sinking with engine
I literally got so fed up and frustrated, after two consequtive evenings of trying to do this on my own, i sat there shouting come on ya B****** pumping the peddle like a loony all connected up. both feet on occasions
until instead of sinking (on the last pump obvioulsy).... i nearly gave myself a hernia....
your luck with this may differ.
bench bleeding done by the book is just shifting a large amount of fluid through the cylinder, out through the port and back into the cylinder resevoir again via a loop back pipe which you can make if you fancy it.
The flow out of the ostensibly disconnected port is quite a high volume, every pump shifts a piston-bores worth directly out of the port. so it brings air with it easily.
i'm guessing my ranting and stomping did something similar although the volume of liquid moved would be tiny in comparison with the system all standard and sealed
you can discount it being a problem at the back. by adjusting the the rear brakes on all the time
this should raise the peddle "hard" point slightly under your foot becasue you just made it so the rear wheel cylinders have no where to expand to, shoes are pressed to drums already.
slightly higher peddle but still sinks... confirms its most likley a problem up front
messy option
spanner on the pipe nut at the master cylinder (do each port in turn)
get somone to stand. both feet on the brake
arm yourself with a good wad of rags around the master cylinder
undo the pipe nut very very slowly. As it oozes get the peson in the car to warn you about the peddle getting to the floor Nip up the fitting just before they hit the floor or if you mis it when they hit the floor, provided they don't then let the peddle up.
what you have done there is made a halfway house from bleeding and bench bleeding the master. low resistance due to your deliberatly created leak at the brake master cylinder port promotes flow which might bring some air with it.
one last thing......
Does your master cylinder (Aussie and some1990s US)
OR
does your brake distribution block (most US cars from mid 60s to 70s)
have a wire connctor on it ?
If so this is a brake warning light connector
under that connctor is a spring loaded ball bearing tipped switch, it runs on a spring loaded and contoured piston that sits between front and rear brake circuits.
if one circuit has a leak the piston moves to block off the circuit with low pressure leaving the other operational.
on US cars it moves back again once the issue is fixed or your remove your foot from the brake.or it should, they can get stuck...!
on later master cylinders and Australian cars it Locks into the position leaving 1 circuit blocked, the contoured sliding piston has a lock groove at each end on these cars and is held locked by the ball bearing end of the switch.
the way to get the piston back into the middle when stuck or when locked (as appropriate to the style) is to undo the locking nut around the connector and allow it to pull out a fraction of an inch, this allows the switch section underneath to float out a bit, so it doesn't press on the piston . The Piston is spring loaded and should ping back to the "Both circuits work" position.
a gentle tap with a cloth wrapped hammer or rubber mallet may also achieve the same outcome
Its worth checking that your bleeding action has not pushed this piston into a position it has never been before... and its got stuck
if one circit is blocked and the other still has air it may promotethe behaviour you see
if all else fails there is the nuclear option.. (i read about this i didn't do this)
submerge the whole master cylinder with its lid off in the cheapest dot 3 you can find
pump it and pump it until no air comes out
put lid on whilst submerged
block off each port with a felt pen cap or the red port block off caps it came with when new in the box
retrieve it, clean it up real good with degreaser brake cleaner or fairy liquid
re mount it and try again.
With the lid on the fluid should not come out and you can connect up the pipes and do the nip up the pipe nut style bleeding mentioned above
you really don't want to be doing this.....
Dave
until instead of sinking (on the last pump obvioulsy).... i nearly gave myself a hernia....
your luck with this may differ.
bench bleeding done by the book is just shifting a large amount of fluid through the cylinder, out through the port and back into the cylinder resevoir again via a loop back pipe which you can make if you fancy it.
The flow out of the ostensibly disconnected port is quite a high volume, every pump shifts a piston-bores worth directly out of the port. so it brings air with it easily.
i'm guessing my ranting and stomping did something similar although the volume of liquid moved would be tiny in comparison with the system all standard and sealed
you can discount it being a problem at the back. by adjusting the the rear brakes on all the time
this should raise the peddle "hard" point slightly under your foot becasue you just made it so the rear wheel cylinders have no where to expand to, shoes are pressed to drums already.
slightly higher peddle but still sinks... confirms its most likley a problem up front
messy option
spanner on the pipe nut at the master cylinder (do each port in turn)
get somone to stand. both feet on the brake
arm yourself with a good wad of rags around the master cylinder
undo the pipe nut very very slowly. As it oozes get the peson in the car to warn you about the peddle getting to the floor Nip up the fitting just before they hit the floor or if you mis it when they hit the floor, provided they don't then let the peddle up.
what you have done there is made a halfway house from bleeding and bench bleeding the master. low resistance due to your deliberatly created leak at the brake master cylinder port promotes flow which might bring some air with it.
one last thing......
Does your master cylinder (Aussie and some1990s US)
OR
does your brake distribution block (most US cars from mid 60s to 70s)
have a wire connctor on it ?
If so this is a brake warning light connector
under that connctor is a spring loaded ball bearing tipped switch, it runs on a spring loaded and contoured piston that sits between front and rear brake circuits.
if one circuit has a leak the piston moves to block off the circuit with low pressure leaving the other operational.
on US cars it moves back again once the issue is fixed or your remove your foot from the brake.or it should, they can get stuck...!
on later master cylinders and Australian cars it Locks into the position leaving 1 circuit blocked, the contoured sliding piston has a lock groove at each end on these cars and is held locked by the ball bearing end of the switch.
the way to get the piston back into the middle when stuck or when locked (as appropriate to the style) is to undo the locking nut around the connector and allow it to pull out a fraction of an inch, this allows the switch section underneath to float out a bit, so it doesn't press on the piston . The Piston is spring loaded and should ping back to the "Both circuits work" position.
a gentle tap with a cloth wrapped hammer or rubber mallet may also achieve the same outcome
Its worth checking that your bleeding action has not pushed this piston into a position it has never been before... and its got stuck
if one circit is blocked and the other still has air it may promotethe behaviour you see
if all else fails there is the nuclear option.. (i read about this i didn't do this)
submerge the whole master cylinder with its lid off in the cheapest dot 3 you can find
pump it and pump it until no air comes out
put lid on whilst submerged
block off each port with a felt pen cap or the red port block off caps it came with when new in the box
retrieve it, clean it up real good with degreaser brake cleaner or fairy liquid
re mount it and try again.
With the lid on the fluid should not come out and you can connect up the pipes and do the nip up the pipe nut style bleeding mentioned above
you really don't want to be doing this.....
Dave
The Greater Knapweed near the Mugwort by the Buckthorn tree is dying
Re: Brake pedal sinking with engine
My brakes are awful currently - no vacuum - but I hadn’t considered the little safety switch. Thanks Dave - will check it out.
"Cum homine de cane debeo congredi." Woof.
Current Charger status - "Working !"
Current Charger status - "Working !"
Re: Brake pedal sinking with engine
Just to finish off this thread: after trying most of the ideas recommended above, I eventually got the car transported to my local American specialist garage.
They've managed to improve it a bit, so the brake pedal is now higher, although without being able to get it all the way back up to the level I remember it at before changing the master cylinder.
A few points came out of this work - mostly just improving on the work I'd done:
- They bled the system with a pressure kit, and got a little more air out
- They adjusted the actuator between the booster and master cylinder out a little (lengthened it)
- They put a small amount of further adjustment into the rear brake drum pads
The combination of these small improvements has got it to a point where the car stops well, and is locking up fully towards the end of the pedal travel. The garage noted how effective my booster appeared to be.
I'm still a little mystified about why the pedal doesn't behave identically to before the master cylinder change (given that it was, or appears to be, an identical replacement) but at least stops the car well now, so I'm happy to leave it there.
Thanks again for all those that offered comments - hugely valuable as always.
They've managed to improve it a bit, so the brake pedal is now higher, although without being able to get it all the way back up to the level I remember it at before changing the master cylinder.
A few points came out of this work - mostly just improving on the work I'd done:
- They bled the system with a pressure kit, and got a little more air out
- They adjusted the actuator between the booster and master cylinder out a little (lengthened it)
- They put a small amount of further adjustment into the rear brake drum pads
The combination of these small improvements has got it to a point where the car stops well, and is locking up fully towards the end of the pedal travel. The garage noted how effective my booster appeared to be.
I'm still a little mystified about why the pedal doesn't behave identically to before the master cylinder change (given that it was, or appears to be, an identical replacement) but at least stops the car well now, so I'm happy to leave it there.
Thanks again for all those that offered comments - hugely valuable as always.
1970 Plymouth Cuda 383.
Re: Brake pedal sinking with engine
Glad you got it sorted.
A small note of caution: you can get immediate results lengthening the push rod to the M/C from the Servo, but after repeated hard stops the fluid expands and you may find you are back into binding again. I have seen cars locked stationary due to this.
It may be worth carrying sufficient tools to ensure you can drop the M/C and back the rod off until you are absolutely confident that the correct and optimum adjustment has been achieved, and that it is reliable.
All the best..
Pete.
A small note of caution: you can get immediate results lengthening the push rod to the M/C from the Servo, but after repeated hard stops the fluid expands and you may find you are back into binding again. I have seen cars locked stationary due to this.
It may be worth carrying sufficient tools to ensure you can drop the M/C and back the rod off until you are absolutely confident that the correct and optimum adjustment has been achieved, and that it is reliable.
All the best..
Pete.
Pete Wiseman; Cambridge.
Mopar by the grace of God
Mopar by the grace of God
Re: Brake pedal sinking with engine
Good news
getting there...
Dave
getting there...
Dave
The Greater Knapweed near the Mugwort by the Buckthorn tree is dying
Re: Brake pedal sinking with engine
All good info. Thanks.
"Cum homine de cane debeo congredi." Woof.
Current Charger status - "Working !"
Current Charger status - "Working !"