Sticking Brake Piston

[byroncobra]

Dear Font of Knowledge

The Problem:

My R32 rear brakes are dragging to the point where they are smoking hot after a 10 minute drive. Both sides. When I lift the rear the hubs can't be turned by hand.

My Attempted Solutions:

1. Replaced wheel bearings thinking the heat and noise was coming from them. This was incorrect....

2. Overhauled calipers with a new set of piston and dust seals. Still got the problem...

3. Triple checked that the handbrake shoes were not dragging on the drums.

Next thought was to check if there is retained pressure in the lines holding the pistons in after release. I released the bleed valve and voila a squirt of fluid and I could turn the hubs again.

So:

Do I pull off the master cylinder, can the piston stick in one direction? It was new when installed in the car, only done a couple of thou km.

The brake line may be blocked? Only in one direction? Back brakes only?

Your thoughts greatfully appreciated

[07cob]

Greg

Given that you have looked at the obvious at the rear of the car, I would be looking at the MC next. If you do the same test (release bleed valve) on the front, is fluid released? If not, could it be that the proportion valve is sticking in position, keeping pressure in the rear lines? Would be strange on a new unit though.

However you fix it, imagine how much better your car will go with the back brakes free!

Geof

Geof

[byroncobra]

Thanks Geof,

I'm sure it's only the rear as there's no problem spinning the front wheels. I don't expect a pressure build up there. I'll try it tho.

I remember a Harrison exclusive fix on the MC to get the back brakes more effective, I can't remember what I did though!

I'll pull it off....cant think what else it could be.

I have been looking for excuses for slow lap times......

[gjkrv8]

I was thinking that if its both sides have residual pressure then maybe MC (not pistons). Do you have a bleed valve on the MC to do the test you did at each Brake Caliper/piston?

Cheers Gregg

[Gaz64]

Master cylinder pushrod is too long, covering the compensating port in the master cylinder.

[byroncobra]

I was thinking that if its both sides have residual pressure then maybe MC (not pistons). Do you have a bleed valve on the MC to do the test you did at each Brake Caliper/piston?

Master cylinder pushrod is too long, covering the compensating port in the master cylinder.

Gary
This MC has been fitted for a while.....no problems up until recently.

Gregg
No bleed at MC, only way is to crack the line to either front or rear.

[sideshow]

if master cylinder has two pistons maybe the second is getting stuck
if they have 2 pistons normally closest one to firewall does fronts and furthese does rear

if its single piston then if that sticks u will have alot of freeplay in pushrod so thats easy to tell

when rear brakes are locked have u tried cracking the front bleeder on the mc
if this releases rear brakes it then points more towards master cylinder holding pressure
and not the lines or rear calipers

did it just happen out of the blue
or after u did a pad change

[07cob]

Originally Posted by byroncobra

I remember a Harrison exclusive fix on the MC to get the back brakes more effective, I can't remember what I did though!

Did you make a change to the MC from the original? There was talk of removing an o-ring from the proportioning valve to equalise the pressure and provide more grip at the rear.

Geof

[byroncobra]

yeah i did that geof, but early in the build but this problem is recent.

Gaz got me thinking, re the actuating rod on the booster. Mine doesn't have an external adjustment just a bolt that screws through the pedal bracket to adjust the freeplay and brakelight switch. Perhaps it's moved in, shortening the pedal throw and thus actuating the brake. Anyway, I've adjusted it ready to test......
however I now need to bleed the brakes after cracking them earlier and i won't have a helper till tomorrow so watch this space......

Jim, no recent pad change, it was during the trackwork at the Nationals that I first noticed the heat and i just put it down to heavy braking, but it's all the time now.
It's a Ford XD single piston MC, no obvious freeplay that you mention. If the test tomorrow fails I'll try your suggestion to crack the front brake line

Thanks Fellas

[renovationinnov]

check you have at lest 25 mm play at the foot plate on the pedal
then the bad news ,,, i had the same problem in a mates car i borrowed . the piston cracked just near the return spring [internal] & wouldn,t let the rears release so in the middle of no where i vice gripped the rear line and drove back home !!
so try pulling it off after purchasing a new one from repco
sorry ROB

[Merv and Sharon]

Is this the one you have? My Ford (wagon) one has a small screw underneath that can be released and needs to be correctly positioned to ensure the piston does not stick.