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Discussion Starter · #1 ·
Help please!

The car is an Australian Chrysler VE Valiant. This has the 273 cubic inch small block and is more or less a Dodge Dart saloon built under licence in Australia with some local panel changes.

Brake master cylinder is a fresh reconditioned unit.
VH44 remote brake servo on opposite side of the engine bay was bought supposedly reconditioned from Australia, but failed (brake fluid leaking into the vacuum diaphragm housing). It was sent to a reputable brake servo reconditioner in the UK who specialises in classic cars, old Jags etc.

The fault:
When you start car up from cold, brakes work fine for a while. As engine warms up (with tickover speed increasing slightly as it warms up) what you see is that as you use the brakes, the front disc brakes start to bind, they work fine but do not release fully at the calipers, pedal comes up normally. As engine warms up to correct operating temp, the front brakes bind even more.

Investigations:
If I gently lift brake pedal to maximum limit (thinking maybe return spring not working or something) it makes no difference.
If I turn engine off, front discs remain bound.
If I then disconnect vacuum hose from remote brake servo, brakes still remain bound.
If I THEN (vacuum hose disconnected, engine now off) press the brake pedal once, normally and release it, there is a squelching noise from brake servo as if brake fluid under pressure is being released ??? back to reservoir - and brakes free up again (so not a sticking caliper - both have also been rebuilt).

The vacuum line to manifold is not blocked, and if I suck on manifold end of the hose from brake servo, it will hold a vacuum (diaphragm not torn). No loss of brake fluid from reservoir.

Not sure what the problem is. It seems to be in the (twice rebuilt) remote brake servo I think.

Any ideas? It makes the car almost undriveable, at times it will not even go to third gear but stays in 2nd as it assumes I am going up a steep hill!
 

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If its a new master that would normally run drums all around then you need to remove the residue valve that feeds the front as that keeping the pressure on the discs ,if single circuit then still remove or check return spring is on or strong enough to pull pedal back fully .
 

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Per Pauls reply above first, ensure anty internal residual pressure valve is working ,and rated, correctly.

I've had recon remote servos where they have stuck internally . Simplest method to test is make a new brake line to run straight from m/c to next female fitting after servo 'out' port . Bleed through and see if fault still there . If it isn't then servo needs looking at again. You can road test but CAREFULLY as brake effort will be raised considerably.
 

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One very simple classic possibility.

When you put it back together you didn't leave any slack at all in the master cylinder pushrod, so when it heats up it can't return fully, or at least not till the relif port inside the cylinder is uncovered.

Screw the master cylinder pushrod adjustment back one full turn from when it rattles in the piston, and then try it.
 

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I don't know anything about a VH44 remote servo but if anything like a Lockheed unit the symptoms point to the reaction piston sticking.
On the Lockheed servo, some models have two seals on the reaction piston and these oppose each other and cause the piston to stick in the on position.
Removal of one of the seals usually cures the problem.
 

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bypass the servo and try again. this will at least tell you if the fault is with the servo.
I agree that you should try bypassing the servo.

A quick test would be to clamp up the vacuum hose and then after pressing the brake pedal a few times give it a try with a slow test drive.
If the problem is gone then the servo is at fault.
 

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Discussion Starter · #10 ·
Thanks for the advice everyone.
i) Master cylinder pushrod has plenty of residual slack when pedal is in up position (actually it is not even adjustable). If changing to wrong brake fluids it has been reported that master cylinder seals can swell so much one of the side holes can become covered even when the master cylinder piston has fully returned - however this seems to be a really rare fault.
ii) Flex hoses all new.
iii) It could be a residual pressure valve incorrectly fitted in the circuit to the front calipers, preventing the remote servo depressurising properly by keeping the air inlet valve from closing again when foot taken off brake pedal (which is supposed to restore vacuum equally to both sides of the big servo diaphragm so it doesn't push any more). I checked this by connecting a plastic pipe to the master cylinder outlet with the barrel of a syringe held upright on the other end full of brake fluid. The fluid drains back towards the master cylinder even when the syringe is held only about 20 cm above level of the master cylinder. Therefore I figure if there is a residual pressure valve mistakenly fitted in there, it must open at a really tiny pressure. I concluded that there isn't one therefore, which is good.
iv) Therefore I took the air valve part of the remote servo apart after doing hours of research on this on the web. There is a small side piston on the remote servo that opens a valve that lets air into the rearmost side of the big rubber diaphragm, so letting the vacuum on the front side of same diaphragm then draw it forwards to apply the power assistance. This tiny piston is about 6mm diameter and has 2 seals on it. It is moved by hydraulic pressure coming in from the master cylinder and opens the air inlet valve. However t can stick open and this has been reported by owners of several old cars with remote servos! Plenty of reports on various forums of old TR's suddenly getting stuck with brakes jammed on (a bit worrying really).
I also get the impression that on a well used car this piston would tend move more freely than on a little-used one.
v) I looked at mine and it does look new (good as servo supposedly had been rebuilt) but was pretty stiff in its little bore, maybe precisely because it IS new and not worn at all. The spring that supposedly helps return it seemed much weaker than required. One solution is to get some very fine wet and dry with WD40 on it, hang the bore facing downwards, and very gently hone the bore slightly until the small air valve piston moves a little more freely up and down the bore, taking time to this a little tiny bit at a time.

I have done all the above reassembled everything and bled the brakes (yet again). Another test drive awaits.

John
 

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Hi John,
Back in post#7 I mentioned the problem with 2 seals on the activation piston.
Removing one of the seals usually solves the problem.
Having 2 seals is not a good design as the seals tend to oppose each other and there is a risk that the second will eventually go dry and cause sticking.

The spring you describe as "much weaker than required" does not in itself return the airvalve piston to it's off position.
It's the vacuum acting on the rubber diaphragm that pushes the piston back.

Good luck with the test.

Regards Denis.
 

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Help please!

The car is an Australian Chrysler VE Valiant. This has the 273 cubic inch small block and is more or less a Dodge Dart saloon built under licence in Australia with some local panel changes. ...
Hi XenonJohn, can't help with brake fault, but as a fellow VE owner, I'd be interested to see a photo or two of your car. I'm aware of three others (than mine) on the road in the UK at the min (although there could be more).
 

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Discussion Starter · #13 ·
I did in the end hone the bore of the air bleed valve small piston as it was sticking in there pretty tightly.
After putting it all back together it now works fine.
If you run car into garage, braking a little, and turn off engine, the brakes will then be left in "on" position at front, however a good shove of brake pedal with ignition off releases everything so not a problem.
Thanks for all the suggestions.
John
 
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