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View Full Version : Testing brake master cylinder - blocking output ports?


twr7cx
31st May 2020, 04:35 PM
Have a brake master cylinder that I suspect may be faulty, brake fluid passing the internal seals causing the brake pedal to slowly keep going down when there is vacuum boost assistance. Is it feasible to disconnect the two outlets, plug them and then try - if it still happens it would indicate fault master cylinder, but if it no longer occurs then must be something further down the line?

Any idea on the thread type for the master cylinder outlet ports on a Discovery 2? Looks like maybe M12 x 1.0?
Max Headroom 2.3m
31st May 2020, 10:27 PM
Looks like maybe M12 x 1.0?
Yep. I had to make a new brake line from MC to ABS modulator on a 2003 Defender which used M12 x 1.0 unions. I measured them against the D2 unions and they are the same. Makes sense -same ABS (more or less), same MC fabricator (Lucas/TWR).
twr7cx
1st June 2020, 09:35 AM
Awesome, thanks for the confirmation. I didn't have any M12 x 1.0 bolts or thread testers in the workshop (only x 1.25, 1.5 and 1.75) but the sizing looked right just with finer threads. Will go try local ABS, Enzed, etc. to see who has something suitable to bloke the ports (as don't want to damage the flange face at the bottom).
discorevy
1st June 2020, 10:11 AM
you could use bleeder nipples , most of the japanese 4x4's are the same m12x1.0

If you have a brake flaring tool , you could make a loop

If you can pump the pedal up quickly with engine running and it holds then you have air in the system

But if you have bled the system , including the modulator , ( inlets as well as outlets ) and you are sure there is no air , then if there was a problem other than the master cyl it would show as a leak and you'd be losing fluid
twr7cx
3rd June 2020, 04:19 PM
Local ABS was able to help me out with blocking off the ports. They didn't have any plugs for the M12 x 1.0 thread but had M10 and suitable adapters - this was actually very beneficial as I was able to unscrew the plugs a bit to bleed the air out while a second person pushed the brake pedal. Part numbers below for anyone else interested in the future.

https://i.ibb.co/64Jymyb/IMG-3725.jpg (https://ibb.co/64Jymyb) https://i.ibb.co/TBczjmt/IMG-3726.jpg (https://ibb.co/TBczjmt)