Strange brake pedal behaviour

[vernon.taylor]

Salut

I seem to remember reading about this somewhere on the forum but I can't find it.

Anyway, twice now, just after breaking to a stop the brake pedal has gone hard and risen firmly. Both times I had stopped at traffic lights and both times there was a moment of panic so I couldn't think/analyse. Both times, too, the pedal became normal after a few seconds.

Everything in the braking system is refurbished and there is at least one non-return valve on the bulkhead.

Any ideas, please ?

@+

Vernon

[Europatc]

Salut Vernon,
the only similar experience that I have had was when the servo was on the way out, but if its all new then you would hope that would eliminate that source. Sorry cant be of more help
good luck
Stuart

[Galwaylotus]

Definitely servo related. More than that I can't say for certain.

[661]

My recent thread was similar-ish. Appears servo related and I'll be taking it out , hopefully tomorrow.

[andywatts]

Salut,

I have done a bit of work on the servo, and I've got the Girling manual for units 2A (early) and 2B (which is original for my 69 +2).

The fault-finding guide is not very helpful - it says:-
- hard pedal - lack of vacuum
- pedal pushes back - inlet and outlet connected the wrong way round or 'fault in unit'.

So I think you have to follow the 'fault in unit' assumption! The diagram of the unit shows a piston and a sleeve between the high pressure and low pressure side, with a ball valve arrangement to allow HP fluid to return to the master cylinder when you take your foot off. Perhaps that arrangement is not working, so that the high pressure fluid is not vented. If your brakes have a bit of air in them there could be quite a bit of expansion on the HP side when you take your foot off the brake, and thus quite a bit of fluid to come back to the master cylinder.

Anyway, as others have said, this is probably a full strip-down job. I believe that recon kits for the Girling units are no longer available (though Classic Car Automotive can apparently still recon some units). You might have to change to a Lockheed unit.

Good luck!

[reb53]

I had exactly this with a rebuilt unit.
When it was rebuilt the "experts" had binned all the springs etc and replaced them with the ones in their kit, ( probably Indian......).
Suffice it to say the servo has never gone again, properly, and has now been replaced with a Lockhead, which works fine but remember to install it at the angle they say in the instructions.

Ralph.

[rgh0]

The attached gives a good description of the Girling Mk2 unit

http://volvo1800pictures.com/document/G ... d%202B.pdf


It is most likely the piston assembly has developed a leak allowing high pressure fluid to get back to the master cylinder side from the caliper side when the brakes are applied. This will push the pedal back up as the vacuum system continues to apply assistance and pushes the servo piston down the bore. Due to the vacuum piston rod on the master cylinder side the volume on this side is less and thus the displaced fluid pushes the master cylinder piston back due to the need for space to be created for the volume of fluid.

A strip down and check and most likely a rebuild of the unit is required to fix the piston assembly leak.

cheers
Rohan