Servos keep failing

Rorie · Mar 19, 2020

Replaced my servo (correct type) and master when i was doing my rebuild. Within first month i had to replace the servo. The brake pedal pressure kept building while driving until the brakes eventually were on. Tried everything - adjusting rod in servo etc. Changed the servo and problem went away instantly.

Replaced with Bearmach one (other was £40 Brit part) and got 10 months out of it with great brakes. Rebuild my front callipers and a week later, over night my servo assist has gone completely. I Was trying the usual trick of building up pressure and turning engine on, but the pedal was solid after a short distance of travel so pressure seemed to already be there.

Put in my old servo to check and instantly had servo assist again, but of course, its back to doing that weird thing of gradually building pressure as i drive until the brakes are locked on.

Called LR direct regarding the bearmach one failing, they said they’ve never seen or heard of this before so it needs to go for testing....

So questions are;
1) First servo, is this a known failure issue where it builds pressure over time?!
2) With my second one failing (not providing servo assist), could there be an issue elsewhere causing the servos to fail? I.e. the master?
3) why don’t i loose pressure in the brakes that would then require me to pump them and build the pressure (for the servo test)? Maybe due to limited travel with my new pads?
4) what am i missing? If something was wrong with the rebuilt callipers, could that cause issue?

nobber · Mar 25, 2020

What engine is it? What did you do to the vacuum pump during the rebuild?

steve2286w · Mar 25, 2020

Is the brake pedal set correctly, if not it might allow air in to allow servo to work when it should not

jamesmartin · Mar 25, 2020

pressure creeping on is usually a too long servo rod,but it can be a failing servo as ive found personally,crap parts are not unusual now and they come in batches,not such an issue with fast moving parts like brake pads but more so with slow moving parts like servos

Rorie · Mar 26, 2020

@nobber I didn't rebuild the engine, so not done anything to the vacuum pump. The hose from the pump is decently creating a vacuum though (the correct amount, I am not sure)

@steve2286w Setting the brake pedal would be with the servo rod, right? In which case it is set correctly, but then as the servo builds pressure, the pedal gets higher (less travel before pressure is on the pedal) until it is basically solid with minimal downward movement. This is with the first servo that I put back in. The second servo, I even removed the locking nut behind the bulb on the push rod so that I could get it wound back into the servo more.... and still the pedal was solid.

@jamesmartin the creeping pressure is with the first servo (the one I put back in). I played about with the servo rod for weeks. It was wound in enough that I had loads of travel in the pedal, then over a drive the pressure would build until the pedal was solid! So I'm not sure if it is the servo rod in that case.
For my second servo, the Bearmach one, I removed the nut on the push rod so that I could get it further into the servo. The travel on the pedal feels right, but it is solid from a start and the servo doesn't allow the pedal to travel down.

I am assuming both servos have failed for different reasons?

Just to make sure I'm working the right parts and there Is no issue with a servo having the wrong length push rod, for example.... its a G reg 90, but with a 300tdi. The pedal box is original (I think, shown on the link below as for 90/110 with type 800 servo),
the servo I am using is NRC4772 Type 80
The master is NRC9529

http://www.expeditionlandrover.info/dual_brake_conv.htm

jamesmartin · Mar 26, 2020

Rorie said:
@nobber I didn't rebuild the engine, so not done anything to the vacuum pump. The hose from the pump is decently creating a vacuum though (the correct amount, I am not sure)

@steve2286w Setting the brake pedal would be with the servo rod, right? In which case it is set correctly, but then as the servo builds pressure, the pedal gets higher (less travel before pressure is on the pedal) until it is basically solid with minimal downward movement. This is with the first servo that I put back in. The second servo, I even removed the locking nut behind the bulb on the push rod so that I could get it wound back into the servo more.... and still the pedal was solid.

@jamesmartin the creeping pressure is with the first servo (the one I put back in). I played about with the servo rod for weeks. It was wound in enough that I had loads of travel in the pedal, then over a drive the pressure would build until the pedal was solid! So I'm not sure if it is the servo rod in that case.
For my second servo, the Bearmach one, I removed the nut on the push rod so that I could get it further into the servo. The travel on the pedal feels right, but it is solid from a start and the servo doesn't allow the pedal to travel down.

I am assuming both servos have failed for different reasons?

Just to make sure I'm working the right parts and there Is no issue with a servo having the wrong length push rod, for example.... its a G reg 90, but with a 300tdi. The pedal box is original (I think, shown on the link below as for 90/110 with type 800 servo),
the servo I am using is NRC4772 Type 80
The master is NRC9529

http://www.expeditionlandrover.info/dual_brake_conv.htm

thats not the rod that is the servo as you thought

tottot · Mar 26, 2020

It does look like you have two duff servo's. A basic check for the vacuum pump is remove pipe from servo and with engine running put your thumb over the end, it should stick and not drop away under its own weight.

Servos keep failing

Rorie

Active Member

nobber

Well-Known Member

steve2286w

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jamesmartin

Well-Known Member

Rorie

Active Member

jamesmartin

Well-Known Member

tottot

Well-Known Member

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