Sticking Brakes

[robertstevenson]

Hello,

I have a problem with the brakes on my 1965 Series 1 4.2 FHC, which is the common problem of sticking brakes or more recently brakes that will fully jam on when driving and won't release.

A few weeks back the brakes suddenly applied yet I hadn't touched the brake pedal. The pedal was rock hard. It was quite a scary experience. I can only guess that having pressed the brake pedal just before joining the the A1 that it it had slightly stuck on before fully jamming less than half a mile further along and fortunately for me next to a turn off.

To get things moving again I disconnected the vacuum hose from the engine (manifold) and put a sealed bit of hose over the exit from the manifold itself.

After this brake pedal had movement and the brakes worked again although I suspect without servo assistance given the effort required to stop the car. I was able to drive home( about 3 miles) without any further problems.

I've bookmarked many of the very useful articles on this forum and have tried many of the suggested tips one of which was the disconnection of the hose from the manifold which I mentioned earlier. I'm very grateful for the advice that has been posted.

So far I have replaced the check valve and the plastic reaction valve and it's rubber seal. This has had no effect. Following a post on this forum I attempted to remove the small piston from the master cylinder. My wife gently pressed the pedal but the piston did not ease or pop out as I expected. I suspect it is jammed in there solid.

There was evidence of red rubber grease around the hole to the piston as the servo was overhauled a couple of years before lockdown but I suspect over time the piston has now fully seized in the cylinder body.

I'm thinking a new brake master cylinder is the way forward given it has already been overhauled.

If I'm missing anything obvious I'd be grateful for any advice before I order the part.

Many thanks.

Robert

[tim wood]

Sounds like you have found the culprit. The sticking piston is common but I’ve never heard of one refusing to come out.

Since you’re going to remove the master cylinder anyway you could disassemble to try to find the problem ?


Tim

[MarekH]

Vacuum assist exists because there is atmospheric pressure on one side and vacuum tank pressure on the other side of the big rubber diaphragm in the booster. Vacuum assist is removed when there is atmospheric pressure on both sides of that big rubber diaphragm in the booster. What you are describing is an inability to purge the vacuum in the front of the booster.

The little plate inside the end of the master cylinder which pushes that tiny piston may have quite sharp edges, so if it is misaligned, it can catch and not move back. It sounds like whoever applied the red grease only knew half of what they needed to know.

Disassemble. Remove any burrs and sharp edges on that plate. Clean out any tiny metal fragments. Regrease with red grease, reassemble and rebleed.

kind regards
Marek

[robertstevenson]

Thanks Tim and Marek for your replies.

It looks as though I'm looking in the right area now but I'm still minded to replace the servo and to strip and hopefully service the old item at my leisure.
regards
Robert

[MarekH]

Why? It's got nothing to do with the servo.

You can confirm that by unplugging the vacuum hose going out of the front of the servo and that'll instantly readmit air into the front of the servo, removing the differential pressures to thus release the brakes.

kind regards
Marek

[robertstevenson]

My mistake Marek. I had meant to say master cylinder rather than servo.
Kind regards
Robert