Front brakes locked on

[Dannyb]

Went out to get petrol for the run to Duxford tomorrow when my brakes locked on. (brake peddle rock hard) The only way of freeing them was to let a little brake fluid out of both frot calipers.

As soon as I operated the brake  peddle they locked back on. So after freeing them again I drove home on the handbrake. I also noticed the heat generated has destroyed one of the front wheel bearings.

The car is fitted with a remote servo, so I think It's either the servo or the master cylinder causing it. 

Whats the best way of faulting it. Do I bypass the servo and try it, any suggestions welcome.

Danny 

[glang]

This is almost certainly a problem with the servo air valve which is keeping the vacuum in the applied state rather than sealing the port to atmosphere correctly. Try removing the vacuum connection pipe from the servo and hopefully, once its vacuum has dissipated, the brakes will operate correctly albeit without servo assistance...

[Dannyb]

Quoted from glang-

This is almost certainly a problem with the servo air valve which is keeping the vacuum in the applied state rather than sealing the port to atmosphere correctly. Try removing the vacuum connection pipe from the servo and hopefully, once its vacuum has dissipated, the brakes will operate correctly albeit without servo assistance...

Thanks I give that a go tomorrow.

Danny

[ferny]

Did the backs lock on as well? If the fluid was "locked", why did you need to do both fronts to release the whole system?

 

I'd be suspecting the front flexible hoses and saying it's bad luck both went at the same time.

[Dannyb]

Quoted from ferny-

Did the backs lock on as well? If the fluid was "locked", why did you need to do both fronts to release the whole system?

 

I'd be suspecting the front flexible hoses and saying it's bad luck both went at the same time.

It was only the front that was locked. You have given me something else to think about now ☹️

 

[drofgum]

Danny,

I suspect you have dual circuit brakes with only the front on the vacuum booster. Booster sticking on won't hold the rear brakes in that case.

Best of luck,

Paul

[glang]

Yes Paul but that wouldnt explain why both front calipers had to be vented to release them unless, as Ferny suggests, each has suffered a colapse to its hose inner liner.

This is a known phenomena where a flap of the failed rubber liner acts a non return valve so keeping hydraulic pressure on the caliper pistons but it would be incredible bad luck to have it happen twice....

[ferny]

Quoted from drofgum-

Danny,

I suspect you have dual circuit brakes with only the front on the vacuum booster. Booster sticking on won't hold the rear brakes in that case.

Best of luck,

Paul

Fair point! Thought I was reading a post in the Herald section but now see it's for a 1500 Spitfire. Discount my previous post as I think the 1500 is spit as you say. 

 

Or, don't they have a proportioning valve which causes similar symptoms? Never worked on 1500 brakes.

[glang]

Still cant see why you would need to vent both calipers to release the pressure from a faulty servo unless theres a dual one fitted....

[Dannyb]

I removed the dual brakes 17 years ago and went for single system so the servo was on front and back.

I never got a chance to look at them today but will keep you posted on how I get on.

Don't suppose I can actually test if it's the hoses without replacing them.

 

[Dannyb]

Today is the first chance to fault the braking system. 

I Fitted 2 new brake hoses on the front. Bled the brakes and the front lock on again.

I have not started the engine so the servo has not pressurised the system. It only locks the front rears are OK. Let out a small dribble from the caliper nipple and they release OK. 

So is it the master or can it still be the servo. 

Remember this is a single line arrangement and no pdwa valve.

Danny

[RobPearce]

So... when you apply the pedal, the front brakes lock on. The rears don't. You have a servo but it's between the master and the first union. Once locked, the front brakes can be released only by bleeding off some pressure from them.

Please confirm all the above is accurate.

Please answer the following:

- When releasing pressure, do you need to open both front bleed nipples or is only one sufficient?

- In normal operation, do your rear brakes actually work?

- Is there anything else fitted in the brake hydraulic system?

[Dannyb]

I now have all 4 wheels off the ground and all wheels run free . Apply the brakes and all wheels lock OK. Release brakes nearside front Rock solid, offside  front just a bout turn it. 

Rear brakes are binding but not as much as fronts. Bleed offside front and all wheels release OK.

All there is in the system is single line master going to servo and out to union then off to the brakes.

Danny

[glang]

This article is interesting as it describes symptoms very similar to yours although I dont know if it applies to your particular type of servo....

http://www.head2head.free-online.co.uk/Rover/servo.htm

[Dannyb]

That is interesting.. I will read it again later but it does seem the same. Next job I suppose is take the servo off and connect a new brake pipe between master and union connection. 

Danny

[Dannyb]

Ok I've bit the bullet and taken the servo out, then made a connection between the two brake pipes. Bleed the front brakes only a they stick on after pumping the pedal, not as solid as before but then there is probably still some air in the system. Ondo bleed on calaper and all free up. 

Funny thing is as I'm bleeding the brakes using a one way valve the pedal sometimes went hard.

So not sure to buy a new master or try a rebuild kit. 

Danny

[RobPearce]

Right, so the testing you've done today says that the problem is, in fact, the whole system remaining pressurised (and the "apparently only the front wheels affected" is merely because your rear brakes aren't that great anyway). The fact that the problem remains with the servo removed means it's not a servo problem, and it's likely it's the master cylinder, although it's not a common failure mode.

I think I'd be tempted to strip the master down and see if there's anything obvious. If it looks too bad, buy a new one, otherwise try a rebuild kit.

If you couldn't see any reason for the problem, and the rebuild kit doesn't sort it, I would have a think before buying the new master, as it's just possible the problem is something obscure. A bit of swarf in one of the unions could, I suppose, move around as the brakes are applied and released, and happen to sit over the "upstream" orifice on release. That would do it. Also, that same foreign object could, during brake bleeding, occasionally sit on the downstream orifice, thus causing the pedal to "sometimes go hard".

[Dannyb]

Thanks Rob,  I will strip it first and see if there is something obvious as you say. 

WWatch this space 😊

[Dannyb]

Thanks Rob,  I will strip it first and see if there is something obvious as you say. 

WWatch this space 😊

[Dannyb]

Ok I took the master off and gave it a good clean. Couldn't check the seals as it looks like it's unserviceable. There seems no way of getting the piston out. 

Put it back and bled the system and all works fine. I will now get the servo back on and report back.  The only thing I can think of is some time ago I put adjustable operating rods on the clutch and brake to take  up any slack. I have now put the original one back on the brake. Although it all looked OK maybe this was not releasing properly and held pressure on the master,  who knows. 

Danny

[RobPearce]

Getting the piston out is a matter of removing the circlip (which I don't see in your photo) then tapping, prodding, possibly fitting a grease nipple and pressurising, filling with plus-gas and leaving overnight (piston downwards) or various other methods. It should just pop out of its own accord but never does if the cylinder's actually ever been used.

Still, if a clean up to that level seems to have fixed the problem let's just hope whatever was up has been cured. Non-standard pushrods are a possible culprit.

[Dannyb]

I have just noticed with the engine ticking over, if I pump my brake pedal the engine speed will. Rise a few hundred revs and dies down when I release the pedal. Is this normal, I can't say I've noticed it before. 

I have can't get a hard pedal after bleeding the brakes three times. 

Danny

 

[glang]

well each time you operate the servo, on release, the air on one side of its diaphragm has to be sucked out and of course this goes into the carb manifold where it can affect the mixture slightly. Its not normally noticeable but if you do it repeatedly it might be sufficient to change the engine revs. I suppose there could be a leak on the vacuum side or the servo air valve isnt sealing completely but then I would have thought you'd see the effect all the time....

[Dannyb]

Yeah that makes sense. I have also noticed it only happens on choke. When warm it is very slight and would not be niticed. 

Just got to keep on bleeding I suppose to get firm up the pedal. 

Danny