Girling servo experts needed!!

[2tmike]

I've been trying to sort the miserable brakes on my S4 and have hit a problem (and fortunately not a wall !!)
Initial issue was high braking effort and a 'dead' pedal feel. The master cylinder looked original and the servo is a Girling Mk2A. All flex pipes are new and although not perfect, the calipers are not seized.
I firstly replaced the master cylinder with a new item and this very slightly improved matters but I could then feel the brakes 'hanging on' and shortly later had a severe loss of fluid which could have only gone into the servo.
A repair kit was sourced and the servo stripped. Indeed the missing fluid was in the vac chamber and the air control valves were covered in black gunge. I carefully stripped and cleaned everything and actually all the metal parts and vac piston seal were in perfect condition as far as I could see, no corrosion or scoring.
The servo went back together without problems and all seals were greased up with the supplied red rubber grease. I checked every part for free movement as I assembled it and didn't fully tighten the vacuum chamber bolts fully until I was reasonably sure that the piston rod was centred well.
After refitting the servo and bleeding the system the pedal felt fine so I gave it a short road test and thats where it all went wrong....the initial feeling was that the brakes were good and servo assistance was there for the first time but after a few firm applications the brakes started to lock on and eventually driving was impossible. After waiting 5mins or so, pumping the pedal with and without the engine running the brakes freed off enough to drive home. On the return trip the same thing happened.
Can I say I do not want to run the car without a servo but just would like some pointers to know where to look in repairing it.
Thanks, Mike

[elanfan1]

If it?s an S4 I bekieve you should have a MK11B servo. There are ones that look similar but have a different diameter (7 inch I think) whereas the correct one is about 5 inches. I?m stripuggling to remember exactly where it is but there?s a ratio stamped on the end of something somewhere (sorry very helpful I know!), if you?ve got the wrong ratio one that?s not going to help.

Job?s comforter mode off/

[2tmike]

Thanks but it is a Mk 2a for certain, flanged vac chamber cover with piston and leather piston seal

[elanfan1]

I?m saying you?ve got the wrong servo.

[2tmike]

I'm not sure Lotus would agree with you, that's how it came from the factory!!!
Anyway, it's of no interest in solving the problem
Thanks anyway

[ncm]

Hi Mike, there are fault finding tests on the second page of this : http://volvo1800pictures.com/document/G ... d%202B.pdf

( I have tried to attach the pdf but the computer says no!)
Brian.

[Chrispy]

I'm having similar issues with my Mk2a servo.

Mine will provide no assistance until I'm braking moderately and then it will kick in with a lurch. Once it's on it's nice and powerful. Occasionally it will stick on, but a bootful of brake releases it immediately.

When putting mine together the control piston seemed a bit firm to get in, and the body had been sleeved before.

Speaking to a bloke at work who used to be involved in a braking specialist supplier company suggested that the main vacuum drum part needed to be practically polished to a high shine for it to work properly. When I put a rebuild kit through mine I just degreased it and wiped it out, then greased with rubber grease.

[2tmike]

Hi Brian, Chris,
Thanks for the posts. I've got the Volvo fault finding file but is doesn't directly address the locking on issue. The info in it though is useful. When I assembled the servo nothing felt tight or looked wrong but after thinking about it I can only put the problem down to either the the vacuum piston sticking and holding the hydraulic piston down or the air control valve system not allowing air into the vac chamber when the pedal pressure is released and keeping the vacuum piston applied. Either way it looks like its got to come out again for a close look. Before I do that I might get it to the brakes locked on state and then disconnect the vacuum line downstream of the non return valve to bring the vac chamber to atmospheric pressure to see if that allows the vacuum piston to retract - if so the issue is probably in the air control shuttle or air valves ?

[Chrispy]

Sounds like a good first test before pulling it apart again!

[oldelanman]

This thread on another forum may hold a clue .... second post down. It's a few years ago but may still be relevant.

https://www.britishcarforum.com/bcf/sho ... uum-servos

[vxah]

You are sure it's not the master cylinder keeping a little pressure in the system due to maybe push rod length? Now you have got the servo working again the assistance gets the brakes holding tight?

[2tmike]

Result of a quick test this morning. Brakes not locked on, engine off, pumped brake pedal multiple times, no locking up. Repeated with engine on and the brakes lock on after a number of pumps. With the brakes locked on and engine off , slackened the 4 screws retaining the shuttle valve chamber lid and released the vacuum in the servo. After a short time ( as long as it took to undo the screws) the brakes released by themselves. Repeated this to confirm. So, a bit inconclusive really, looks like a problem with the air control system but after breaking the vacuum the brakes don't release instantly......perhaps a combination of a fault in the air control valves plus a tightish piston in the vacuum cylinder due to the new seal ?

[2tmike]

Servo off the car, stripped to its component parts and I honestly can't see anything wrong with it !!!!
Only option seems to be to reassemble with plenty of grease in the shuttle valve and refit the old vac piston seal to see if that makes a difference......

[vxah]

Did you get the brakes locked on and then slacken the hydraulic pipe from the m/cylinder? A little residual pressure in there will keep the servo on..

[2tmike]

Problem solved....I hope!!
Basic issue was the shuttle valve controlling the air and vacuum valves. I thought to try more grease to ensure it moved smoothly and easily but actually it had the reverse effect - more grease and a very stiff, sluggish valve resulted. Grease removed and lubricated with brake fluid and suddenly it moved easily and quickly. I was also going to fit the old vac chamber seal but that disintegrated as I was handling it so the new one went back on but slightly shortened because as supplied it was 10mm too long and athough it did fit under the leather lip seal I think it was making it too tight. Silicon grease in the vac chamber replaced by moly and the vac piston moved more easily as well. A short test drive and the car stops as it should without any locking on........
Now just the clutch and handling to sort out....))