Saggy Expert brake pedal

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Wookey
Donor 2019
Posts: 210
Joined: 28 Dec 2004, 09:43
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Saggy Expert brake pedal

Post by Wookey »

I have a now 23-yr old 1.9TD Expert and I often have to do some brake-fettling to pass an MOT.

The car hasn't been anywhere since March last year, except moved from drive to garden a few times to make room for building works. I checked it over in the summer thinking it was MOT time but decided that the brake pedal really did go a bit too far down to be convincing and I should try and get to the bottom of it. After reading up here (e.g 'Citroen Dispatch Brakes' viewtopic.php?t=38048, and 'Synergie - sinking brake pedal' viewtopic.php?t=30484), I suspected that my master cylinder may be at fault and I should test it. Then it was winter and far too miserable, and I still had nowhere to go, so only now, with some temps in double figures, did I get round to some spannering.

However the results are confusing, so I come to report and see what people think.

The main issue is that the brake pedal is nice and hard with engine off. If I start it, then the pedal sags about 3-4cm as it should, but if you keep pushing it goes further down. Not all the way to the floor, but further than feels 'good'. From this I conclude that my servo is working as it should. I have found no leaks anywhere, and it all got a good fettle last MOT (july 2019ish). Fresh fluid, a new rear cyclinder, a new (well s/h) compensator valve, some new pipe, full caliper service with new sliders and seals on nearside, and the year before that offside caliper service and new front flexpipes at front both sides.

To test the master cylinder I wanted to block off the ports and see how the pedal worked with the rest of the system disconnected. I tried putting rubber bungs under the brake nuts but that didn't work so I made up a couple of blocked-off nuts -i.e a brake nut with 3cm of pipe, flattened and folded over to make a stub. Putting those in the front two ports (and leaving the back two as-is) the pedal sagged much less - an obvious improvement (presumably despite a little bit of air in that stub?). OK, so one of those pipes is the nearside front and the other goes to the compensator. So this seems to say the issue lies in one of those. (The other two ports are much more of a pain to get to and thus block off and test, and there doesn't seem much need now). So I left the stub in the rear compensator port and reconnected the front nearside. Now the pedal seems to have gone back to sinking too far. Even though this is a very short pipe (1m?). I bled it as some air probably got in during these tests, but nothing seemed to change. It was getting late so I may have been a bit slapdash, but what might be going on?

It seems to me that my master cyclinder is in fact OK, given that blocking off two ports removes most of the sag. But if there really is an issue with the front nearside, where might the 'sag' be coming from? Am I missing something? Might my fairly new flex pipe be 'stretching' because it's too cheap and nasty? Something else?

What would you test next?

I guess I should put up some pics and a schematic of the brake layout, as this may not be totally clear. I can do more checks this w/e but I'm a bit confused at this point.