Servo headache

[Dearg1275]

What the devil is going on here. First the background. 5.5 inch Lockheed servo rebuilt, good bores & diaphragms. Newly restored brake system filled and bled. Engine run with vacuum hose connected. Brake pedal pressed with out engine running to check the brake light switch. Brake lights work but don’t go out when brake pedal released. Pedal rock hard.

Brake lights go out when system cracked open to release the pressure. i.e. not a brake switch fault. Suspect the servo. By-pass servo with master cylinder to three way union brake pipe. Brakes behave normally and brake light goes on and off as it should.

Strip servo and find the air valve control piston free to move. There was a degree of fluid leakage pass the seal ( I shouldn’t have trusted those seemingly good seals after all). The main piston rod was free to move. Again a little leakage into the vacuum chamber. In short I could see no reason for the servo hanging on. My only thought was perhaps some debris had been blocking the return of fluid from beneath the air control piston.

The servo turns out to be a larger bore version 0.7 inch rather than the “S” 5/8 inch but I can’t see this making any difference except longer brake travel before the servo kicks in. So before I put it all back together with new seals and fingers crossed can anyone spot anything I have missed?

To help the brain cells tick over here is the diagram from the service manual.

02EE7244-5E58-4AFB-B0E4-0FC9EB3C7FE7.jpeg

Suggestions to just do without the servo duly considered.

D

[dodge44]

I had this some time ago - in my case the control valve was not releasing the vacuum once the foot pressure was taken off the pedal. Been a while ago now so can't quite recall for sure but *think* the one way valve between the manifold and servo was at fault.

[Peter Laidler]

Back to basics and Dodge has it..... IF.....! Engine not running, the servo plays no part in the braking system (when residual vacuum has been vented of course). If engine not running and system stays in depression/in vacuum and brake light holds on, then one way valve has stuck David.

I read somewhere that the one-way valves are hard to get. As a matter of interest, are they repairable?

[InnoCooperExport]

where in the chain have you got your brake light switch? Normally it comes before the servo, so I'm surprised that even if the servo is keeping the pressure on down the line the switch should release, right?

[Dearg1275]

Back to basics and Dodge has it..... IF.....! Engine not running, the servo plays no part in the braking system (when residual vacuum has been vented of course). If engine not running and system stays in depression/in vacuum and brake light holds on, then one way valve has stuck David.

I read somewhere that the one-way valves are hard to get. As a matter of interest, are they repairable?

I don’t follow your logic here. The one way air valve between the servo and the manifold has one function. It is there to maintain a sub atmospheric pressure in the servo “vacuum”chambers at all times until the servo is activated by hydraulic pressure in the brake line. If it were leaking and the engine not running the reduced pressure in the servo would equalised with the atmosphere.

With the reduced pressure being maintained, courtesy of the one way valve, the servo has a one time chance of action when the engine is not running. Clearly this happened as evidenced by the rock hard pedal with no travel.

where in the chain have you got your brake light switch? Normally it comes before the servo, so I'm surprised that even if the servo is keeping the pressure on down the line the switch should release, right?

The brake pressure switch is in the usual position in the line before the front brakes. This is a single line system and the servo is between the master cylinder and all brakes and the switch.

D

[InnoCooperExport]

where in the chain have you got your brake light switch? Normally it comes before the servo, so I'm surprised that even if the servo is keeping the pressure on down the line the switch should release, right?

The brake pressure switch is in the usual position in the line before the front brakes. This is a single line system and the servo is between the master cylinder and all brakes and the switch.

D

That explains my confusion. I have a split system where the brake light switch is on the PDWA switch on the bulkhead. So between the MC and the Servo.

It's definitely a headscratcher you've got there.

[andy1071]

The size of the cylinder (0.7 rather than 5/8") will affect the pressure in the brake system, so if anything, would reduce pedal travel. -But not enough to notice, so I wouldn't worry about it. (the size of the hydraulic cylinder and the size of the main vacuum chamber each affect the amount of assistance: big vacuum chamber = more assistance; small hydraulic cylinder = more pressure)

The fluid leaks are, of course of concern, and need to be corrected.
The illustration is rather mis-leading, as it doesn't show the 'valve piston' moving, and physically moving the diaphragm, as it should.

Obviously any debris in the servo is bad news, so you need to check. Also check that all the small holes are clear.

From the symptoms you describe, I would suspect the filter/mesh in the air valve body (indicated by 'atmospheric pressure' in the illustration) -any blockage here will cause the problem you have..
-you can temporarily remove it to check the effect...

[Dearg1275]

From the symptoms you describe, I would suspect the filter/mesh in the air valve body (indicated by 'atmospheric pressure' in the illustration) -any blockage here will cause the problem you have..
-you can temporarily remove it to check the effect...

Andy. Thanks for you thoughts. However, (that bogie word) a blocked filter on the air control valve would prevent the servo from activating in the first place as there would be no way for atmospheric pressure to enter and act on the main diaphragm. Equally if it were blocked, it would still not prevent the pressures in all the chamber equalising when the air control piston retracts as hydraulic pressure from the master cylinder is released.

I’m beginning to be convinced that there has been a blockage in the return of fluid from beneath the air control valve piston. Checking it again today and rodding it out with some 0.8 mm wire allowed a lot more light to shine up the bore. It would have taken very little to occlude it. Perhaps that’s the answer. If, once re-assembled, it all works, all we will truly know is that it now works. What we will know is that these servos are tricky beasts and being absolutely sure everything, everywhere is spotlessly clean is a good starting point.

D

[Nick W]

The last servo I fitted worked ,but as you described held on for a fraction after pedal pressure was released.
I put around 120 miles on the car and the problem seems to have virtually vanished.
I did bleed the brakes a couple of times to make sure no air was in the system after putting some miles on it.

[1071 S]

Its a while ago now but I bought a NOS (Lockheed- I think) brake master cylinder a while ago and suffered the same symptoms.

I forget the exact technicalities but the cylinder was missing a bleed hole (or some such) that meant it did not release when the pedal was released. Once you pressed the pedal the car stopped until you cracked a joint in the line to release the pressure.

Pretty sure the cylinder was genuine as it came from an old school (and trusted) brake specialist who still had all manner of dusty stock on his shelves...

Cheers, Ian

[OGW 1082]

Hello David,
I had a booster reconditioned some years ago now that had the same symptoms as you describe.
Rather than sending it back to the shop, I pulled it apart and found that the wrong size (slightly smaller cup ) had been installed.
I put a new cup of the correct size in and it worked perfectly. I don't know why, I'm not that clever to figure out hydraulics when they misbehave. I also had another one done that was resleved and left with some glue in the little drilling for the air control valve. It didn't work at all.
Regards.
Glen

[imack]

I had a similar problem with this valve (circled) sticking after I rebuilt the servo.
I could remove the filter and push the pistback down by hand after brake application and the brakes would release.
My servo was from a Hillman Hunter, I possibly fitted a cooper s repair kit ( it was a long time ago), I've since read somewhere that this is a symptom of fitting a cooper s repair kit to a hunter servo.

[Dearg1275]

I had a similar problem with this valve (circled) sticking after I rebuilt the servo.
I could remove the filter and push the pistback down by hand after brake application and the brakes would release.
My servo was from a Hillman Hunter, I possibly fitted a cooper s repair kit ( it was a long time ago), I've since read somewhere that this is a symptom of fitting a cooper s repair kit to a hunter servo.

Hello David,
I had a booster reconditioned some years ago now that had the same symptoms as you describe.
Rather than sending it back to the shop, I pulled it apart and found that the wrong size (slightly smaller cup ) had been installed.
I put a new cup of the correct size in and it worked perfectly. I don't know why, I'm not that clever to figure out hydraulics when they misbehave. I also had another one done that was resleved and left with some glue in the little drilling for the air control valve. It didn't work at all.
Regards.
Glen

Yes, this is the area of concern. Interesting that the wrong sized cup might make a difference. I have an “S” servo stripped along side this one and although the bore size for the air valve piston and the pistons are the same the seals are very different. The one on the offending servo having a long taper.

2FAD1CC1-C2BB-400B-BFD4-63D5D2830658.jpeg

As said earlier the piston was not seized in the bore at all so it is hard to fathom why this should make a difference. It will be interesting to see what shape the piston seal is in the repair kit that is on its way from North West Classics. Peter there was very helpful in coming up with the correct size main seals. If you didn’t know most seals have the part number on them (unless they are tiny like the ones above).

With luck the seals will turn up today.

D

[Dearg1275]

Just to conclude this. The seals arrived in swift order. Great service. The servo went back together with no issues making sure the drilling for the air valve piston was absolutely clear. It now works as it should.

On reflection that very fine drilling to a small chamber is a recipe for trouble. Muck goes in but doesn’t necessarily leave and builds up over time. Given a chance it consolidates in the drilling especially if the servo sits with its air valve upper most.

Let the bairn play with the knife it’ll learn. You are wiser each day if you play.

D