Locked on brakes

#1
I've read all of the excellent advice on this forum, on this subject! - and I really would be grateful for any hint as to what I'm doing wrong. My project car; not been driven for years. It had locked on brakes and having read the advice here I tried releasing the air pressure to the servo - the inlet pipe - which sure enough released the brakes (but then so did releasing air from the white control valve?). So, yesterday I fitted a brand new brake master cylinder. It was the first time I have ever done such a thing, so I'm not saying I got it right, but I followed the WM - although I didn't, as it suggests, remove the heat shield and spacer because a) the last bolt is impossible to get at, and b) I couldn't see the point of doing so. Anyway, today, I bled the system using a 12v device that applies gentle pressure and fresh fluid through the reservoir, and opened up the bleed nipples in turn - got a lot of air out of the fronts, as expected.

Now, all four brakes are locked solid and nothing whatsoever will release them - I've pulled both pipes to the servo, and nothing doing.

What have a I done wrong?
 

harveyp6

Well-Known Member
#2
Have you set the pedal height correctly? Have you released the pressure by slackening a fluid connector? If so does that free the brakes?
 

roverp480

Active Member
#3
I found the brake servo sticking on with my P4 when I didn't use it for about 4 years which meant the brakes locked on, slackening a hydraulic union to the wheels freed the brakes off, , but next application jammed them on again . Eventually after may rebuilds of the servo to try and get the piston moving freely I replaced it . Problem solved.
 
#4
Have you set the pedal height correctly? Have you released the pressure by slackening a fluid connector? If so does that free the brakes?
Thank you Harvey. I measured the height of the pedal as the WM suggested, and set it at that - and played with different heights to see if it made any difference. It didn’t. I’ll try slackening a fluid connector as you suggest. Which one should I try - the rearmost connector to the master cylinder, or perhaps one the connectors to the servo? Thank you again, A
 
#5
I found the brake servo sticking on with my P4 when I didn't use it for about 4 years which meant the brakes locked on, slackening a hydraulic union to the wheels freed the brakes off, , but next application jammed them on again . Eventually after may rebuilds of the servo to try and get the piston moving freely I replaced it . Problem solved.
Thank you. I have the servo as a prime suspect now...
 

harveyp6

Well-Known Member
#6
I’ll try slackening a fluid connector as you suggest. Which one should I try - the rearmost connector to the master cylinder, or perhaps one the connectors to the servo? Thank you again, A
If you slacken the inlet into the servo and they free off then the master is the problem. If they don't then lock them on again, slacken the servo outlet, if they free off then the servo is the problem. If they're still stuck the the problem is at one of the lines to the calipers.
 

jp928

Active Member
#7
I have seen front brakes jam on due to standing unused for a long time, and corrosion on the pistons preventing them from retracting properly. Also hardened caliper seals wont help at all. Recently read a book on the Jag D types that raced with the first dunlop discs. They made sure the pads would last the whole race because the way they were built meant that a pad change all around could take all day. Partly this was because they didnt realize that with the right shaped seals, the seals themselves would retract just enough to release the brake.
 
#9
Right, so I decided the servo wasn’t working. A recon one arrived today, which I bolted in - and one of its two studs is loose and is just spinning round. I’m worried this will allow a leak in the chamber thereby reducing the servo’s effectiveness. Should I be?
 

sdibbers

Well-Known Member
#10
That’s not right. It can cause a vacuum leak and in turn it will affect the engine idle etc. sounds like you need to return it for a sound unit.
 
#11
I'm going to have to bite the servo bullet soon. They're a PITA. :rolleyes:

Thirty six years ago I bought my P5B in Cuffley, from the showroom that used to be near the railway bridge. It's gone now though l still have the car.
 
#12
Just about ready to give this car away to the first taker. Locked on brakes despite new servo and new master cylinder - and now, releasing the air in the servo isn’t releasing the brakes, as was the case with the previous servo. What the hell am I doing wrong?
 

quattro

Administrator
Staff member
#13
That's weird

Take a front wheel off and release the bleed screw, you may want to put a hose on it first.

I'm just wondering if the pistons aren't retracting as the car has been off the road for so long.

If they stay on, your pistons are seized, if not you have a pressure problem.
 
#14
I’ve bled them to death! Seems unlikely all four pistons would stick on simultaneously. And yesterday the brakes were releasing if I relieved pressure at the servo. I’m wondering if it’s possible a brake light switch can act as a non-return valve?
 

quattro

Administrator
Staff member
#16
I'm just trying to get my head around this.

With the engine off, pump the brakes a few times and the pedal should get hard. Are the brakes free now?

Then put your foot on the brake pedal, start the engine and the pedal should go down an inch or so - does it do this?

How do you know that all of the brakes are locked on? Do you have the wheels up in the air?

Richard
 
#17
When I let a little fluid out of the pipe that connects the servo with the master cylinder, the brakes release. Pressing the pedal (engine off) and they lock on again.
 
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