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Clifford Pope · Jan 20th, 2004, 19:58

It sounds like the master cylinder leaking internally. This often happens after any major work even if the fluid circuit has not been disturbed. What happens is that either pressing in the caliper pistons to fit the new pads sends old dirty fluid back up the pipes, or else the pedal inevitably gets depressed further than normally and so runs the seals down on to a section of bore that may be a bit rusty or have dirty deposits. Either way the seals get damaged slightly. The brakes may work but as you have found the pedal slowly sinks to the floor.

I wouldn't bother with trying a used MC - they are quite cheap new from Europarts or German-Swedish.
It will need bench-bleeding. With skill and luck you can replace a MC without letting in any air so not needing to bleed the whole system.
If you let in air near the MC you then get involved in the pain of trying to bleed the rear pressure relief valves or re-setting the dreaded 'octopus'.

Good luck - if you find you do need to replace the MC and need more tips do ask - I have quite a good success record (ie 3/3) of replacing MCs without re-bleeding!

Jan 20th, 2004, 19:58	#4
Clifford Pope Not an expert but ... Last Online: Apr 26th, 2024 12:45 Join Date: Dec 2001 Location: Boncath	RE: Brake pedal on 1978 244DL It sounds like the master cylinder leaking internally. This often happens after any major work even if the fluid circuit has not been disturbed. What happens is that either pressing in the caliper pistons to fit the new pads sends old dirty fluid back up the pipes, or else the pedal inevitably gets depressed further than normally and so runs the seals down on to a section of bore that may be a bit rusty or have dirty deposits. Either way the seals get damaged slightly. The brakes may work but as you have found the pedal slowly sinks to the floor. I wouldn't bother with trying a used MC - they are quite cheap new from Europarts or German-Swedish. It will need bench-bleeding. With skill and luck you can replace a MC without letting in any air so not needing to bleed the whole system. If you let in air near the MC you then get involved in the pain of trying to bleed the rear pressure relief valves or re-setting the dreaded 'octopus'. Good luck - if you find you do need to replace the MC and need more tips do ask - I have quite a good success record (ie 3/3) of replacing MCs without re-bleeding!