Locked on brakes

Demetris

Well-Known Member
#21
I once had the same problem, because i rebuilt the brake master cylinder and fit the end washer (just under the circlip) the other way around, so that the piston could not retract fully.
 
#22
I once had the same problem, because i rebuilt the brake master cylinder and fit the end washer (just under the circlip) the other way around, so that the piston could not retract fully.
It does feel like the master cylinder is pressurising the system even when the pedal is high. I’ll take it off tomorrow and see what I can see. Fitting something the wrong way round wouldn’t be a first for me.
 
#24
The higher the pedal is the more likely it is to hold the brakes on.
I meant when the pedal is released. I assume that the lower it is screwed into the trunnion, the more its spring is inclined to pull the connecting rod back? But if that rearward travel isn’t quite far enough...?
 

harveyp6

Well-Known Member
#27
Pedal height isn't the problem then. Have you tried slackening off the pipes when the brakes are locked in the order that I suggested previously?
 
#28
Yes. Releasing fluid from the pipe that connects the servo with the master cylinder releases the brakes. But so too does bleeding fluid from the pipe that connects the servo with the brake light switch.
 

harveyp6

Well-Known Member
#29
If the brakes are locked on, and releasing the pressure from the pipe between the master cylinder and the servo releases the brakes, then the master must be holding the brakes on. Once that pressure is released then the servo releases because it just works the way that it would when the pressure was released by a properly functioning master cylinder.
 
#30
Thank you Harvey. The finger of blame does seem to point at the idiot who fitted the master cylinder. In my defence, it didn’t seem possible to fit it wrongly - but I have previous for finding imaginative ways to cock it up. I’ll remove it tomorrow to see if I can recognise what I did wrong.
 
#31
I once had the same problem, because i rebuilt the brake master cylinder and fit the end washer (just under the circlip) the other way around, so that the piston could not retract fully.
Demetris, you were uncannily prescient. That was indeed the precise problem.

A before and after shot below. The second photo shows the washer the right way round. Such an easy thing to get wrong! (for hamfisted amateurs like me, anyway).
 

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cobraboy

Well-Known Member
#33
Another problem with the pushrod / washer assembly occurs if the pushrod is moved out of line with the M/C during the pedal stroke. Due to the shape of the mushroom on the end of the pushrod it must be straight in line with the cylinder to allow the piston to come back far enough to release the brake. With the pushrod fitted to a cylinder if you move it to an angle you will see what I mean, the mushroom jacks the piston forward enough to cover the port in the cylinder and the brakes will not release. Had this occur on a car in the past and it took a while to figure out what was going on.
 
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