Sticking front brakes

[44DHR]

I have noticed last year when I used the car as I braked to come to a halt the brakes would occassionally seem to apply themselves and stick on.

We have just been out on a 50 mile run on a pretty hot day and the front brakes started to squeak as they were grabbing on. It appears more squeaking on the left side but if I apply the brakes the squeaks stops for a short while, only to reappear after a few minutes. Arriving home the discs are fairly blue, but not excessively, so I believe they are sticking on and not fully releasing.

Just wanted some guidance on the best logical route to follow in diagnosing where the problem is. The brake master cylinder was replaced five years ago, which was 7,000 miles ago with a new Lockheed unit. I have never touched the servo and the brakes are Coopercraft 4 pots with ventilated discs.

Regards,
Dave

[andrewh]

I can't recall, is your car a 3.8? I have noticed on my car that there is a tendency for the servos balance bar to not completely return to the fully off position. I have also noticed on half a dozen cars there is a small spring hooked over the servo bracket that aids this return, if its fitted of course. Next to that, depending on the pads you have perhaps they are tight in the calliper and need relieving so they don't chatter but are a bit less tight . Finally, of course it could be the fluid is getting hot and expanding for some reason. Just a few ideas, I am sure there will be others along in a minute to add to these.

[44DHR]

Hi Andrew,
It is a 1967 4.2 Series 1.
Regards,
Dave

[caveman]

Caliper pistons may be sticking.

[Mark Gordon]

Sounds like what happened with mine. It turned out to be the small piston in the front of the master cylinder which pushes on the reaction valve. It seems that that piston can stick and not release the reaction valve which in turn causes the brakes to fail to release. The fix is to remove the reaction valve, slide the piston out of the front of the MC, clean and lubricate it with red rubber grease and reinstall. A simple check for the problem is to disconnect the vacuum reservoir line to the intake manifold (plug the line so that you don't have an intake vacuum leak) then go for a careful drive. You will still have full braking function, just without the power assist. (Hard pedal pressure will still lock up the brakes.) If the problem goes away, you know that the problem is on the vacuum side of the system (not the hydraulic side) and can then hunt down the source, usually that piston or the reaction valve itself. If the problem persists, you know it's on the hydraulic side and you can search for that source (i.e. weak and collapsing hydraulic hose, sticking caliper piston, etc.).

[PeterCrespin]

Perfect evidence-based advice. With systems as important as brakes, a vague punt is not really good enough, IMO, though it's nice that everyone tries to be helpful.

[caveman]

Or not, Peter.

[44DHR]

Mark,

Thank you so much, this is exactly the evidence-based advice I was seeking.

I think it is unlikely to be a collapsing hydraulic hose as I run braided hoses, but I will start on the master cylinder. When I replaced the master five years ago, it came with a new reaction valve. I then sent off the old master cylinder to be re-sleeved in stainless to keep as part of my Continental Touring spares pack. I took the option for the re-furbishers to rebuild the unit - so I do have a fully serviceable unit unit ready to fit if needed.

Part of the problem may be lack of use, as although I have covered 7,000 miles on the new cylinder, most of that was on two continental trips and in the last two years I have done less than 450 miles in each, so things could start to be sticking. I now remember this sticking piston behind the reaction valve on here before, so this will be my first easy starting point on the presently fitted unit.

Many thanks again for all responses.

regards,

Dave

[JEP41]

Hi Dave,

Just a thought I had a similar problem with my own cars a couple of years ago and called Andy at Hutsons to order a replacement master cylinder. Andy suggested I install a spring at the front and sent me one to assist pushing the shuttle back. It worked a charm and no further problems. Might be worth a call to Andy?

[christopher storey]

John : if it is not the reaction valve, then your next port of call might be the servo shuttle . I had endless trouble with my new rebuild servo with it sticking on, sometimes for as long as 24 hours ! I rebuilt it again with copious red rubber grease, and this cured the problem . Before going to that trouble , however, you could try dissolving a good helping of red grease in fluid and flushing it through several times to both the left hand brake cylinders. The problem seems to be that modern dot fluids do not have the lubricity of the old fluids, and only quite modest "stiction" in the servo shuttle is sufficient to overpower the spring in the vacuum chamber and prevent the shuttle returning fully. Incidentally, a lifetime's supply( 1lb jar) of red grease is available from the Morris Minor centre for about a tenner !

[44DHR]

Thanks one and all. I am now fully armed with my 500gm tin of Red Rubber grease and looking forward to resolving this issue.

I will start with checking through the integrity of my vacuum hoses and then move onto the likely culprit of the the reaction valve and its actuating piston in the Master Cylinder. Following a bleed through the brake system I can then look to the servo shuttle if problems prevail. I do have a heat shield fitted to the frames to protect the servo by the way.

Reading back through the Forum and especially posts from Chris, I now see this is a recurring issue and hopefully this post will assist others in the future and increase the knowledge of our systems, current traits of brake fluids and the benefits of Red Rubber Grease.

regards,

Dave