Synergie - Sinking brake pedal

Gammy leg · Unread post by **Gammy leg** » 09 Apr 2010, 19:40

Chaps,

Thought that I detected a bit of graunching from the front brakes so have just replaced pads with new Ferodo's. Old ones were not that bad after all but would have needed a change shortly so no harm done.

After replacing I took the bus out for a test drive but the pedal now sinks to the floor under pressure. It almost feels like I've loosened one of the bleed nipples but I havn't touched them and there is no fluid leaking from anywhere. There is resistance when pedal is pressed and it takes about 2 - 3 seconds to drop completely. It does also seem to 'pump-up' when pressed repeatedly but will sink under constant pressure. Oddly when the engine is off and the servo vacuum is exhausted it seems to hold it's own.

Any suggestions???

Gammy

Unread post by **Peter.N.** » 09 Apr 2010, 21:27

Could be that the master cylinder is leaking internally or could be that the pads are not a good fit to the disc, in which case it should improve as they bed in.

Peter

Gammy leg · Unread post by **Gammy leg** » 09 Apr 2010, 21:53

Thanks for the reply Peter. I have considered the master cylinder as the culprit but don't know why it should decide to fail at exactly the same time as the pad swap. Could it have something to do with forcing the caliper pistons back into their bores when fitting the new pads??. Could moving the pistons back have caused any air in the system ( I suspect that it did need bleeding ) to get locked in the pipework or master cylinder?

I have done this job countless times over the years but have never encountered this particular issue before.

I will go out first thing tomorrow to buy a bleeding kit. Sods law, off on an overnight trip this weekend and need to get this sorted before work on Monday so time is limited.

Gammy

citronut · Unread post by **citronut** » 09 Apr 2010, 23:12

what mileage has it got n the clock Gammy, as peter says it could be the master cylinder, because after fitting new pads you have to pump the pedel to get the pistons back out, in doing this the master cylinder pistons are travleing up the cylinder bore further tha they usualy travle, this can damage the seals when travleing in un/po;ished bore

regards malcolm

Gammy leg · Unread post by **Gammy leg** » 10 Apr 2010, 00:46

102K on the clock. I'm hoping its just air in the pipes. Taking the master cylinder off on Sunday afternoon after a long drive doesn't really appeal

Gammy.

handyman · Unread post by **handyman** » 10 Apr 2010, 11:34

Hi Gammy, first thoughts are internal leaking in the master cylinder, especially on that mileage, but it could also be a sticking caliper piston or a caliper not sliding on the pin, if you have a single piston caliper.

Did you clean off any crud around the dustshield and the caliper when you fitted the new pads? Are your discs in good order, with no wear lip, as this may also cause a brake 'delay'.

Was the wear on the pads even? If not , this may point to a warped or cracked disc.

Handyman

Gammy leg · Unread post by **Gammy leg** » 10 Apr 2010, 12:15

OK,

Weekend away now cancelled. Need to get this fixed somehow.

I bled the brakes this morning with no appreciable difference, ie the pedal slowly sinks to the floor when pressure applied with engine running. Pedal will 'pump-up' but ultimately sinks under constant pressure. When engine is off, the pedal will hold at about half distance to the floor.
Double checked seating of new pads and they all look OK to me and no visible leaks anywhere.
I have phoned around all my local factors this morning ( incl GSF ) but none carry a master cylinder as a stock item. Prices range from £100 - £160 + vat.
There are 2no outlet pipes running from the master cylinder, presumably one feeds the rear and one feeds the front. At some point there must be a splitter to supply each wheel cylinder. Could this be a possible source of the problem? Also the vehicle has ABS if it makes any difference.

Any pointers would be MUCH APPRECIATED. I need to get this sorted by Monday if at all possible.

Thanks

Gammy

MJM · Unread post by **MJM** » 10 Apr 2010, 12:27

I have heard of master cylinder seals being damaged by caliper pistons being retracted to fit new pads. It may be a bit late for you now but the usual advice is to slacken the bleed nipple and retract the pistons with it open, allowing the fluid to flow out of the nipple.

I think the fluid being forced back causes the seals in the cylinder to become "reversed" in some way.

handyman · Unread post by **handyman** » 10 Apr 2010, 12:56

Hi Gammy, contrary to the idea that the seals in the master cylinder can reverse or the bore wears in different places, this is all a bit of hookum. The piston in the cylinder travels over the same range of the bore as all wear is taken up in the brake caliper.

It may be possible to get a seal kit for the master cylinder, but it can be a bit of a faff fitting, as you will need to get the unit out to fit them.

Have you looked for air leaks in the servo circuit, as this can give similar symptoms? Might be worth checking that the one way valve between the engine and servo is not blocked, as they can get oil contamination.

Have you checked the rear brakes, in case you have a leak of fluid there, especially if you have drum brakes on the rear? What about the flexible lines? Two front, two back, at least. I have known these to take on the dimensions of a small balloon if the pipe is failing!

The two pipes coming out of the cylinder serve the dual brake circuits. One supplies fluid to the nearside front and offside rear wheel, the other the contrary.

The ABS system has no effect on fluid at the wheels. It all happens at the control valve, might be worth checking for leaks there.

Hope some of this helps.

H

Gammy leg · Unread post by **Gammy leg** » 10 Apr 2010, 23:11

I went back to square 1 and removed, inspected and re-fitted the front pads and could find nothing wrong. Everything is working as it should and no leaks visible anywhere.
Next I took off the master cylinder and dismantled as best i could. Couldn't get access to the pressure bores or piston seals though - wasn't able to fathom how to get into the last part of the cylinder - so can't comment on their condition. There was no evidence of leakage past the final seal into the rear of the cylinder though, which should mean that there should also be no pressure drop, or am I talking rubbish??
Re-assembled and refitted then each wheel cylinder bled starting farthest away from the master cylinder working back in turn to the nearest. I Did get a fair bit of murky, creamy gunk out of one line though, but now all pipes are flushed through with clean DOT4.

Agree with you Handyman re: piston travel, the piston always returns back to it's original starting spot when the brake pedal is released as it is spring loaded to do so. Pushing back on the brake cylinder piston will not move it, let alone into unfamiliar terratory.

Is there any other component in the system that could be causing this problem or is the concensus of opinion still the master cylinder?

Gammy

Unread post by **myglaren** » 10 Apr 2010, 23:19

Could it be one of the flexible brake lines bulging? They do deteriorate after a while and lose their ability to withstand the fluid pressure. Perhaps removing the caliper slider has flexed them more than usual and caused them to begin breaking down?

Unread post by **Peter.N.** » 11 Apr 2010, 09:10

myglaren wrote:Could it be one of the flexible brake lines bulging? They do deteriorate after a while and lose their ability to withstand the fluid pressure. Perhaps removing the caliper slider has flexed them more than usual and caused them to begin breaking down?

..or the servo leaking - I assume they still use them.

tonupteabag · Unread post by **tonupteabag** » 11 Apr 2010, 09:26

not knowing what car i'm guessing at this but does it have a rear brake compensator i know this hasn't been disturbed but there is a slim chance it has air in it
it does sound like a rear wheel cylinder from the symptoms described but as its discs all round are the rear caliper seals weeping is fluid in the reservoir dropping
just ideas that have probabley been mentioned but ya never know

tonupteabag · Unread post by **tonupteabag** » 11 Apr 2010, 09:31

having just read through again you can test the flexis by clamping one in turn then pressing the brake pedal the defective one (if it is) will soon show itself as the offending pipe will no longer be able to swell as its clamped so the pedal will stay hard instead of sinking

Gammy leg · Unread post by **Gammy leg** » 11 Apr 2010, 10:46

myglaren wrote:Could it be one of the flexible brake lines bulging?

There are only 2 flexi's on the system, one either side at the front. Have not noticed any bulging when bleeding etc but will double check just in case.