Brake servo

[ETTony]

Team, let me tap into your collective knowledge again.

My brakes do work, nice and even F&B/ L&R. Can lock the brakes.
But the pedal is quite hard and needs a good push.
I am suspicious that the servo is not helping .

Stop engine, evacuate servo res by pushing pedal.
Foot on brake, fire engine. The pedal does not sink.
I do get various clunks from the servo unit when i use the brakes, but maybe this is a reaction from the slave cylinder hydraulics ?

Checked vac hoses are correct, checked vacuum tank check v/v, reaction v/v renewed, servo unit rebuilt, slave cylinder rebuilt, master cylinder renewed.

Removed reaction valve and looked for movement of the intermediate piston which (should) operates the reaction valve. No movement. This is the small piston in the centre.

I feel a need to strip the new master cylinder and endevour to free the int. piston
Or is there some where else i should look first ?

[mgcjag]

Hi Tony...yes the small piston should move....wrap a rag around the end and operate the pedal it should pop out..if you dont use the rag to catch it then it will fly out and get lost......its operated by a small metal flap at the end of the cylinder....Steve

[abowie]

Easiest way to check to see if you're getting servo assist is to disconnect the vac line from the manifold (plug it to stop air leak) and drive the car and see if the pedal is noticeably different.
The small shuttle pistons can jam; this usually leads to the brakes sticking on. Remove it as described above, slather it with rubber grease and put it back in.

[ETTony]

Thanks Steve, thanks Andrew,
Job done, with confidence from your posts I hoofed on the pedal and the piston moved out, lubed and now the pedal lifts on starting.
Once the roads dry I'll wizz around the block.
Our forum rocks.