Hard Brake Pedal after master cylinder change?

[spasimodo]

I had brake problems with my 172 (2), the pedal being soft and needing a sharp tap to 'set' the seals, as when a master cylinder is on its way out. I've changed the master cylinder and a rear nearside caliper at the same time. Now the brake pedal is absolutely solid on start up and I'm unable to bleed the brakes. Is this a servo hose or servo problem (servo is corroded on one side)? I have done a search and most hard pedal problems seem to indicate servo problems.

Any suggestions/comments welcome.

Thanks

Eric

 

[spasimodo]

So I've tested the vacuum from the inlet manifold and that appears to be OK, but I don't know if I would get hard pedal if the vacuum was OK but the servo not working. The other question is, is the aftermarket master cylinder I've fitted responsible for the problem. This means removing the new one and replacing the old to see what result I get.

Suggestions welcome.

Thanks.

 

[JB21]

Have you check the vac pipe coming from the servo isn't kinked behind the engine, they can even melt...

 

[spasimodo]

I've swapped masater cylinders and swapped the new one back (no change) and now I'm going to use a pressure pump to bleed the brakes. Can't see why I would get a hard pedal if there's fluid in the system even though it's not completely bled. I'm thinking the servo is knackered, but still not sure.

Suggestions welcome.

Have you check the vac pipe coming from the servo isn't kinked behind the engine, they can even melt...

Thanks for your input.
All pipework is uncompromised - vacuum present at the one way valve on the servo with no backflow.

My issue is that the brake pedal remains solid, but not only that, when the master cylinder is inserted into the servo, the piston is depressed out into the master cylinder (spring resistance when tightening the master cylinder nuts). This means the brake pedal is fully released and the pushrod is fully withdrawn from the servo, but, on the other side the master cylinder piston is depressed into the master cylinder meaning perhaps a mechanical disconnect between the two. Presently my suspicion is that the master cylinder has not aligned centrally on the brake pushrod, which is why I can feel the master cylinder piston being pushed back into the master cylinder. I think this this is because the brake pushrod is not fitting inside the master cylinder piston sleeve. Instead it is fouling on the edge. A quick look at the old master cylinder which I swapped in and out again today confirms this. On the lip of the old piston there are thread marks in one place which indicate pushrod misalignment. A new servo is £200 (not yet checked all prices) and fitting requires the removal of the accelerator pedal from inside the footwell and then the removal of the clutch cable and then the removal of brake and clutch pedals to withdraw the whole unit and the servo from the car. It's a PITA job so before embarking on it I need to understand what's causing the hard brake pedal. So tomorrow it will be master cylinder out again andsee if it can be re-aligned back in.

 

[Usr117063]

So the pedal is firm when the mc and vac lines are disconnected? Don’t quite understand about the push rod off centre but there is pedal travel in the servo before the push rods move.

 

[Usr117063]

And you don’t need to remove it like that, it comes out as one peice, forgot but the servo stays attached aswell even, is not that fiddly really, can be all out in 15 mins or less, I have no sound deadening so quick bolt access, and I use a socket drill

 

[spasimodo]

Problem Solved - thanks for all your input.
The pushrod end articulatedslightly and would not sit straight hence when the master cylinder piston end was introduced the apperture could not be made to allow the pushrod end to enter the piston cavity and was fouling each time. It needed someone to fully depress the brake pedal to bring the pushrod end to the fore of the servo allowing the servo piston to be aligned. Then it was just a case of coordinating the relaease of the brake pedal with inserting the master cylinder into the servo body.