Brake sticking question

Oldskoolrob

Active Member
#1
Hi, Tonight for no apparent reason my brakes stuck on. There was zero pedal feel like the master was glued in position. Managed to pull out of the intersection and turn her off. After a while I started again and drove home using gears and park brake to get home. At home the problem didn't reoccur. Is this a common fault? What am I dealing with here.....?
 

mrtask

Well-Known Member
#2
Faulty air valve in the servo? I've had that lock up my brakes before. If it locks up again, slacken off the union on the hard brake line from the 'T' piece (where the brake warning switch is) to the servo a little, with a rag underneath to catch any brake fluid, and then nip it back up. If that frees up the brakes again, you've found the issue. I hope I remembered that right! It was some years ago now.
Afterthought! Maybe don't fiddle with your brakes based on my poor description of a possible diagnosis unless you're very confident in your wrenching abilities! Hopefully somebody actually trustworthy will comment in due course.
 
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Oldskoolrob

Active Member
#3
Facebook suggested a collapsing vacuum hose or faulty one way valve on the manifold....mine doesn't have a one-way valve, just straight through. Servo is only a couple of years old. Car was being used semi-regularly for club runs and had no issues until I put it on full rego - now the gremlins start coming!!

Is there a specific one-way-valve vacuum fitting for the manifold to the brakes? I'll replace the hose. My engine DOES give (what I consider) high vacuum and I was decelerating using the engine at the time, if this helps diagnose an issue? I don't know if it's relevant but it also runs a Holley, not SU's.
 

quattro

Administrator
Staff member
#4
I'm pretty sure the one way valve on mine is in the plug that fits into the servo. I can't really see why a collapsing hose or faulty valve would cause the brakes to stick on though.
 

mrtask

Well-Known Member
#7
The air valve on the servo can be fitted with new seals in situ without having to overhaul the rest of the servo.
 

Phil Robson

Well-Known Member
#8
How about a collapsed flexi hose to one of the callipers?

Foot pressure may still apply the brakes, but ‘ambient’ return pressure isn’t enough to release the brake pistons in these situations.

You can’t check the potential problem without removing the hose & seeing if it blows through easily. Once they’re off however, you may as well replace them as they’re cheap enough.
 

jp928

Well-Known Member
#9
After such an event check the front wheels for temperature - I had a TC with very hard brakes and no power - front caliper pistons were rusty and not retracting - wheels got quite hot!
 
#10
I had the same as JP after mine was laid up for a couple of years waiting for the lt77 to be rebuilt and fitted, rusty pistons were the culprit. On the initial test run of about 1.5 miles the drivers side disc got hot enough to glow!
 

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roverp480

Active Member
#12
If the whole braking system feels solid at the pedal I would be more inclined to look to a stuck piston in the Master cylinder or servo ,as the brake fluid is not able to return . I had this on a P4 servo, only way to release the pressure was to release a union. Not enough play in the MC push rod will also result in the same symptoms.
 
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