Brake Refurbishment

[themetallian]

After purchasing my 1971 P5B Coupe last year, I've been working away on some of the many many things I need to do to get the car roadworthy again and back up and running, hopefully to see the light of day this Summer perhaps. The first job was a complete brake system rebuild. Many years of the car sat unused with contaminated fluid had left callipers and cylinders seized, brake pipes and reservoir corroded and everything was in a very poor unloved state.
After quite some hours work, I've now replaced all the brake pipes with nickel/copper and new flexi's front and rear, rebuilt the front callipers, new rear cylinders, new master cylinder, reservoir, new servo, discs, pads, shoes and given everything and good overhaul and lubrication. Time then came to bleed the completely empty system and I've tried my best with a air compressor pressure bleeding kit which I thought would enable me to complete the task by myself. Unfortunately, after bleeding, once the brakes are applied (1 or 2 pumps first time to get the pressure up) the brakes stay locked on all round and only release when I undo 1 or the bleed nipples.
Appreciate is difficult to say but what might the cause of this? I thought I had done everything OK but obviously I haven't and have made a mistake somewhere. I even put the old pushrod back on the new master cylinder as I thought this would replicate the original set up. Somehow once the pressure is applied the brakes come up hard very quickly which I thought was a good sign as not spongy at all and meant I had got things right but the fact they don't release means I've got something or things wrong. Any advice on likely stupid errors I might have made in carrying out this brake refresh would be really appreciated. I've run out of knowledge and experience and now have a car I can't move to even take to someone to look at what I've got wrong. Frustrating after many hours and much expense to sort the entire braking system so any advice and guidance would be really much appreciated as I don't know what to do next to try and resolve the problem(s). Many thanks.

[enigmas]

It may be a brake booster fault causing the issue of locked on brakes.

Certain brake boosters (eg. PBR and perhaps others) will lock the brake system once the engine is started due to a fault in a vacuum port. With PBR boosters it is usually a split/tear or opening in a vacuum rubber transfer elbow. The split allows atmospheric pressure to enter the system. This will cause the brakes to lock-on. Replacing the elbow or repairing the "fracture" so that it is not open to atmosphere fixes the issue.

I've encountered this issue on 2 vehicles, one using a PBR booster and another on a 1960s truck fitted with an aftermarket brake booster.

Usually disconnecting (then plugging) the main vacuum line to the brake booster will allow the brakes to both release and function...but without boost assistance.


PS. Just bleed the brakes normally (deep six the air compressor method) with the help of an assistant and using a jar of brake fluid and an appropriate length of clear PVC tube.

** If you don't know how to do this...just ask. It's a very straight forward method. No fancy equipment required.

Other Possible Screw-ups Causing the Issue.

If it's not the brake booster causing the issue then it might be the mastercyl setup.
I presume that you are using the original style mastercyl that the Factory fitted, and that the pushrod actuating the mastercyl has not been altered with regard to how it engages the mastercyl.

If the pushrod "engagement" to the mastercyl has been changed/or altered from factory standard then there is a possibility that the small fluid transfer ports within the mastercyl are being covered.
This then blocks the fluid from returning to the mastercyl reservoir once pedal pressure is released.

If this is the cause....then back off or remove the mastercyl pushrod from the pedal linkage. This will allow the mastercyl pushrod to fully return home and release any internal pressure in the hydraulic lines.
If properly adjusted there should be a small amount of free play at the brake pedal and pushrod BEFORE it activates or moves the brake mastercyl hydraulic piston assembly.

* For the moment, don't over-adjust the rear drum brakes causing them to lock on. When adjusted properly...the brake linings will just be heard "lightly rubbing" against the internal drum surfaces.

[MK IA Norway Viking]

It could be that the piston in the master brake cylinder is sticking, and not moving out when you release pressure on the brake pedal. Seems to me that your braking system is otherwise fine since it is only the master cylnder that has capability of locking all four wheels.

So, take out the master cylinder, place it in a vice and operate the piston manually - observe for stickiness; it should move smoothly

[lagain]

If it affects all four wheels at least you can narrow it down to the master cylinder or the servo.
One of our members had an issue with a new remanufactured master cylinder, but that was no brakes at all !
I have a Morris Minor that has had a brake servo fitted and some times that sticks and the brakes stay on, but just for a few seconds before releasing, so I would suspect one of those.

[djm16]

Is the wavy washer underneath the piston seal in the MC missing?


I echo the advice about the length of the MC pushrod too.

[Ken Nelson]

Check June 2018 "Lockheed Type 7 Servo" topic also. My problem was a plugged air filter for the brake booster unit.

"Ten or more years ago when rebuilding my P5 3-litre Coupe I had trouble with delayed/slow release of the brakes. I had replaced the Girling servo with the later Lockheed servo and finally solved the problem when I replaced the air filter in the servo. I presume it was blocked enough to restrict the flow of air. Did you replace this seemingly innocuous little part as well? Is it possible that is still some of the problem you are experiencing? My brakes have worked fine ever since doing that."