Sticking disc brakes on both sides (Cuda).

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slumbers
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Re: Sticking disc brakes on both sides (Cuda).

Post by slumbers »

Had a similar problem a few years ago when I modified a Ford Capri with vented discs etc when fitting a V8 engine .The recommended Brake servo was a Capri Mk2 type with Land Rover master cylinder. Every time I took it out for a test drive after about 6 pedal presses the car ground to a stop with smoke pouring out of the front brakes . I couldnt move the front wheels at all and ended up releasing brake fluid from the front brake lines to recover the car .Turned out I had the brake servo pushrod adjusted too far into the master cylinder .This will allow vacuum pressure to continually apply the brakes .Its important as the other guys have stated that there's a clearance between the servo rod and master cylinder plunger face otherwise the servo valve never shuts off so you always have vacuum applied
Last edited by slumbers on Wed Aug 10, 22 8:54 pm, edited 1 time in total.
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Steve
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Re: Sticking disc brakes on both sides (Cuda).

Post by Steve »

Interesting thread. I had a similar issue with my brakes (drums) and found an easy way to check and test the pushrod was to shim the mounting bolts with thin washers until it worked properly. I then took the rod out and adjusted it by the thickness of the combined washers. I only did this because I didn't trust myself to be able to get the rod back to the original setting if it wasn't this that was the issue. Worked OK in the end
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petebryan
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Re: Sticking disc brakes on both sides (Cuda).

Post by petebryan »

Hi all, I've now completed most of the ideas and plans from previous posts and I thought it was worth updating you, as I'm still not at the bottom of it yet.

Despite the tests (See earlier postings) indicating the problem was probably above the calipers, as I previously mentioned I decided to get the calipers reconditioned just becausse they were both out and I wanted to rule them out. I started out trying to get the recon calipers from the US as recommended - they are cheap as you say, but I couldn't get any pair from the same manufacturer (checked the big 3 or 4 outlets like Rock Auto, Summit, YearOne, etc), They seem to have supply issues, not sure why. Instead, I sent them to BiggRed in Worcester, who have a good reputation for reconditioning calipers and other brake stuff (they do all the brake work for CarSOS). I got new pistons and seals from the US, and BiggRed did the recon work (including electroplating), and refitting of the seals, and testing etc, and they look great (see photos attached). Worth mentioning that the old pistons that were removed did not look at all bad, just a few light marks from the seal - see photos attached.

I also bought new slider pins and flexible hoses, and then put the calipers back on.

Before I got around to testing the brakes, I compared the old flexible hoses to the new ones by doing a blow-through. The new ones are certainly easier than the old ones to blow through, so I'm certain the old ones were partly failing and were having at least some effect on the brake binding.

However, having refitted and bled the brakes, the binding is back, at least partly. It's not quite as bad as the start where I couldn't move the wheel at all, but it is dragging pretty heavily, and I can only just turn the wheels around by hand. Again, both sides seem to be affected to a similar extent.

I then removed one of the calipers again and used my G-clamp to try to push the piston back into the caliper, but it really doesn't way to go in even with some pretty heavy twisting on the clamp. I guess this is really just confirming what the initial testing told me - that the problem is somewhere above the calipers. This is also consistent with the old pistons that were in reasonable condition when removed.

Next job was to check the pushrod that comes out of the brake booster and into the master cyinder, to check it wasn't permanently engaging the brakes. The various techniques for measuring this are all a bit difficult but I eventually found a way (on another forum post) that works well: I pulled the brake booster push rod about a centimetre out from its position in 'home position' in the booster (it's loose) and then carefully pushed the master cylinder back onto the studs until it was back flush to the booster flange, ie. its start point. There was no resistance at all from the pushrod as I did this, indicating that it's not engaging the master cylinder piston before the pedal is applied, so that's good. I then carefully re-removed the master cylinder, and the checked how far in the booster pushrod had gone back in, and if there was any more travel before it reached its end position - there wasn't - so it feels like it's perfectly adjusted. Hopefuly I've explained that reasonably clearly! Let me know if any of this sounds wrong please.

By the way, I also checked the pedal mechanism in the footwell - seems to have a couple of milimetres of travel before it engages, so looks and feels perfect to my eyes.

So now I'm at a loss as to what to do next. I'm confident the calipers are all good, and the flexible hoses too. It feels to me like the system gets too much pressure in it and doesn't lose that pressure when the brake pedal is realeased. Any ideas how that might happen? Or what I might check (or change) next ?

Any further ideas appreciated as always!
Cheers
Pete
Attachments
Old pistons (removed)
Old pistons (removed)
2022-08-20 10.03.30.jpg (123.24 KiB) Viewed 3309 times
Reconditioned caliper
Reconditioned caliper
2022-08-20 09.59.34.jpg (95.83 KiB) Viewed 3309 times
Reconditioned caliper
Reconditioned caliper
2022-08-20 09.59.20.jpg (129.25 KiB) Viewed 3309 times
Reconditioned caliper
Reconditioned caliper
2022-08-20 09.59.13.jpg (131.06 KiB) Viewed 3309 times
1970 Plymouth Cuda 383.
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Steve
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Re: Sticking disc brakes on both sides (Cuda).

Post by Steve »

Just thinking aloud and not based on experience etc but is it worth disconnecting the vac pipe to the booster whilst the engine is running and see if the brakes free up a little?? :read2:
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MilesnMiles
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Re: Sticking disc brakes on both sides (Cuda).

Post by MilesnMiles »

Some years ago I fitted a new M/C and booster to my Roadrunner. The discs effectively locked on within a couple of miles. I backed off the pushrod and it was always good after that, so along with the refurbed calipers I’d definitely take Matt’s advice on checking the pushrod.
Sorry, just saw your post from Tuesday🙈 about the pushrod.
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Dave999
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Re: Sticking disc brakes on both sides (Cuda).

Post by Dave999 »

master cylinder ??

you are always best off with the master cylinder it had if its the orginal one....i.e guaranteed to be correct bore for your application, with pipes on the correct side and the correct resevoir for a standard look. if you get a replacement see file below

Rebuild

take off
take to bits
ream
new seals and springs
put back together


PITA to do but if everything else is good the one thing that can hold pressure in the system is the master cylinder
rust in the bore of one part of the system, stopping full return i.e the rear...(a set up designed to move tiny volume to tiny wheel cylidners) could leave the front system seals forward enough to block any return from the front.. front brake system designed to shift coniderbaly more fluid with the same stroke for BIG pistons in calipers

doesn't actually need to be rust
if the fluid has become waterlogged way in the past and has been left long enough the combination of brake fluid and water causes the seals to go spongy like microscopic holes in swiss cheese.
water makes its way into the rubber and stays there and when hot expands the rubber causing it to bind.


On another note

is this the proper master cylinder for the car?
if you have a drum drum master cylinder both circuits may have a residual pressure valve for use with drum brakes only
much discussion about relevance these days as modern wheel cyclinders do not need them the old factory orginals did

if you run a residual pressure valve with disc brakes IT COULD , not saying it would, provide a restriction that would stop the pistons being knocked back and hence binding front brakes

undo the pipe into the master cylinder for the front circuit and using a mirror look for a brass insert with a small hole in it down the port

if there is one wind a self tapping screw into it and pull it out, you don't need it for disc brakes.

Dave
Attachments
Moparts_Complete_Guide_to_Mopar_Master_Cylinders_1967-19746.pdf
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Mick70RR
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Re: Sticking disc brakes on both sides (Cuda).

Post by Mick70RR »

I've had a similar problem with the Road Runner. The brakes were not binding but the pedal had become more like a switch. No feel to it, touch it and the brakes slammed on. I checked the brake calipers and they seemed fine. Next time I used it I noticed the brake lights were not going off, I had to pull the pedal back to get them to switch off. The only spring pushing the pedal back is in the master cylinder so I decided to get a new one. Fitted it today and the brakes are back to normal. I've just re-read this thread and noticed Dave's last post, I guess he had the answer all along.
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petebryan
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Re: Sticking disc brakes on both sides (Cuda).

Post by petebryan »

Just coming back to this thread to finish off, which may help some poor soul in the future.

Turned out that the sticking disk pads were due to a fault with the master cylinder, which works fine now replaced, and binding has gone.

Thanks to all that gave advice, and in particular the advice about cracking the brake line at different point and identifiying what part of the system was causing the problem - that's the key to this issue. [I did a lot of work on the calipers as a relatively easy option, even though all the signs from my testing had pointed to something further up the system].

One other interesting point that came out of my local American car garage looking at it later on: they always put a lubricant onto the caliper pins which allow the disk pads to move better, and avoid any drag from that source. A number of people I chatted to had mentioned a small amount of residual drag beiong common, and I wonder if that's the reason for their problems.

Thanks again.
1970 Plymouth Cuda 383.
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Pete
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Re: Sticking disc brakes on both sides (Cuda).

Post by Pete »

Glad you got it resolved.
I have the opposite = Loose Pads and the car suffers from really irritating "Pad knock" until you apply the brakes...
Pete Wiseman; Cambridge.

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Dave999
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Re: Sticking disc brakes on both sides (Cuda).

Post by Dave999 »

I have tended to put copper-slip grease sparingly on pins or claw slider. carefully onthe dragging edges of rear shoes againts back plate and thinly over the backing of the brake pads, anti rattle shims , with a smidge on the z shaped anit rattle spring mount in the middle of the outer pad (which mine have)... is what ive done

makes taking it apart a horrible dirty job as the tacky grease is a dust magnet, but seems to keep everything quiet and moving

might be wrong just because i do doesn't mean you should. dunno... just keep it off the brakeing surfaces...

i once used copper spray gasket when i had nothing else dunno if that was sensible, tacky paint for shim-gasket dressing with slippy coppery stuff in it :) possibly pointless, but needs must, no rattle or squeeling

i haven't died in a nasty collsion yet...

but i admit i might be promoting bad practice :)

Dave
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