Servo Failing On Long Slow Braking

[redgriff500]

Hi guys,

My 95 Chim has a horrid habit of losing the servo assistance (meaning losing 90% of braking) when I slow for perhaps 5 seconds - eg when you see the lights are on red and you gradually slow down.

I've nearly hit the stationary car ahead of me at 10mph as suddenly it just won't stop.

It's absolutely fine in quick hard braking.

I've renewed the pipe to the servo as the old one was cracked and brittle but this hasn't helped.

I see there is another vac feed pipe going into the wing ahead of the servo feed - where does this pipe go to ?

Any likely causes ?

Thanks

[Colin RedGriff]

Not sure where the second pipe goes to, but if both pipes are OK then it might be the servo unit itself

[SILICONEKID340HP]

They are from a Fiesta ,told off a left hand drive but they might be telling Porkys .

http://www.pistonheads.com/sales/3943601.htm




They are Ford Fiesta bur there are several types used in Fiesta's. TVR used one from a LHD car for some strange reason. This is the correct oner for a Griffith Chimaera or S model. I saw an advert of a main dealer now asking nearly 350-400 pounds. I have very limited stock.

[carsy]

They are just Fiesta of the XR2i era.

[SILICONEKID340HP]

Got one in my spares box ..


What a job to change

[redgriff500]

Yes I'd rather not change the servo itself until I'm sure it's at fault.

No fluid loss and I'm told it's extremely rare for servos to fail.

Also it does work for short braking it just seems to run out of vacuum

[BIGMIKE1]

I once had an Astra that would leak air from the servo depending on how much the brake pedal was pressed, so if you pressed hard all ok, but if you pressed light to medium and listened carefully you could hear air leaking at the pedal end. Might be worth getting your head into the footwell and having a listen while pressing the pedal with varying amounts of pressure.

[Quietlybonkers]

BIGMIKE1 said:

I once had an Astra that would leak air from the servo depending on how much the brake pedal was pressed, so if you pressed hard all ok, but if you pressed light to medium and listened carefully you could hear air leaking at the pedal end. Might be worth getting your head into the footwell and having a listen while pressing the pedal with varying amounts of pressure.

Somewhat tricky approaching the lights at 60 mph??

[BIGMIKE1]

Yes probably better to try while stationary and you might also have to run the engine to build servo pressure then turn it off to give you a chance of hearing something whilst pressing the brake.

Quietlybonkers said:

Somewhat tricky approaching the lights at 60 mph??

[blitzracing]

This issue does tie in with your power step at 3700 rpm, it could indicated an air leak in the plenum somewhere or via the servo. You only notice it a low throttle openings when the plenum vacuum is at its highest so any air leak is at its worse, and the ECU is not getting the air reading from the AFM so it backs the fuel right off, so the engine runs lean. Once you get the throttle plate fairly wide open the plenum pressure rises to nearer atmosphere so the air leak becomes much less noticeable. Time for a really good listen for an air leak on idle.

[rigga]

Used to have non return valves fitted to the vauxhalls back in the day when i used to spanner,these would sometimes fail and bleed off the pressure inside the unit and the result would be as you described,hard quick press not enough to dissipate the pressure but a longer slower one would,having said that i have had faulty units over time,mainly down to rusting away where you cant see it.

[redgriff500]

Well I had a listen and couldn't decide if / where there was a leak so I removed the plenum and resealed it - as it didn't look great before.

Idle has never been great but I need it tuning first (removed the CAT etc)

I drove it 5 miles home and deliberately trying to provoke it (with nothing in front of me) it didn't fail once but the pedal still doesn't feel right.

Also the power step is still there.

[mrmr96]

You sure it's the servo and not the master cylinder?

[redgriff500]

mrmr96 said:

You sure it's the servo and not the master cylinder?

Fairly... I suspected the M cylinder and when it happened I pumped the brakes (as I've had this failure before)

But when you clamp off the servo pipe it exactly replicated the issue and it only happens on long slow braking.

[mrmr96]

redgriff500 said:

mrmr96 said:

You sure it's the servo and not the master cylinder?

Fairly... I suspected the M cylinder and when it happened I pumped the brakes (as I've had this failure before)

But when you clamp off the servo pipe it exactly replicated the issue and it only happens on long slow braking.

Wouldn't a leaky master cylinder also perform poorly on extended braking events? The pressure would seep away past the seals?

[redgriff500]

mrmr96 said:

Wouldn't a leaky master cylinder also perform poorly on extended braking events? The pressure would seep away past the seals?

You tend to lose the pedal totally (it goes to the floor) and if you keep pumping it tends to work (in a fashion)

When I asked a mate who sells TVR parts he also suggested servo... he said when the master goes people tend to order a bonnet and wings too.

[V8RX7]

Thought I'd update this as it might help someone else.

It turned out to be a bulging front brake hose.

I found this out when I had to brake hard and I had total brake failure after the brake hose blew.






Fortunately I was on the MOT brake tester at the time.

[Jonny cobra]

V8RX7 said:

Thought I'd update this as it might help someone else.

It turned out to be a bulging front brake hose.

I found this out when I had to brake hard and I had total brake failure after the brake hose blew.






Fortunately I was on the MOT brake tester at the time.

That was a bit if luck !!!!!!

[Sardonicus]

A faulty leaking servo will never ever give you a sinking pedal/excess pedal travel infact the complete opposite.What BigMike described concerning the Vauxhall servo is a very common way of servo failure.

Edited by Sardonicus on Friday 1st February 22:12

[V8RX7]

Jonny cobra said:

That was a bit if luck !!!!!!

Yes very, on the way back from the station is a tight bend at the bottom of a hill in an NSL so I know exactly where I would have ended up if it blew the next time I braked.