How can you tell if a Master Cylinder goes bad?

[1990m3]

I have a lot of pedal travel on the first pump of the brakes, sometimes the brakes barely work at all with one pump. On the second pump the pedal feels normal like there is no problem.

Here's what the car has that are known good parts:

new pads
new rotors
new super blue brake fluid (bled by myself and confirmed by my shop that there is no air in the line)

All parts have been checked with a caliper and are within spec, the rotors easily exceed minimum thickness. There is no vibration in the braking, just too much pedal travel. When I get pressure built up the brakes feel good and stiff and there is no loss of pressure so I believe my brake booster is working properly. All signs point to the master cylinder at this point, but I'd like to know if there is a way I can test it before I just buy another one. Thanks

Dave

[M3 Adjuster]

what happens if you hold the brake pedal? does it fall or stay firm?

it sounds like you have a bad master cylinder though if you are SURE the brakes are bled properly..

I prefer the two man, 13 pumps per corner bleed personally..

you could possibly score a used OEM master cylinder on the cheap from someone who did the 25 mm upgrade

[hardtailer]

Originally posted by 1990m3
I...When I get pressure built up the brakes feel good and stiff and there is no loss of pressure so I believe my brake booster is working properly.....

the opposite behaviour (i.e. pedal sinking to the floor) would not allow the conclusion that the brake booster is faulty!

Only two phenomenons point to a faulty booster:
-high pedal effort. Most easily noticable when stepping on the brake pedal, engine running, car stationary. It will be very hard. --> booster either doesnt get vacuum of the engine or booster itself is broken.

- the booster doesnt hold its vacuum. Start engine, apply brakes and check if pedal feel is normal. Turn engine of and donot touch the brakes for another 2-3 hours. Apply the brakes after that without having the engine run (not in the mean time, not when you apply th ebrakes)

From your description I would actually draW THE conclusion that the MC is ok...

[1990m3]

M3 Adjuster - Brake pedal stays firm if I hold it down, does not fall at all. I am sure that the brakes are properly bled and no air is left in the system.

hardtailer - The brake booster is fine most likely. It holds vacuum and the pedal effort is completely normal. I have a spare working master cylinder I might swap in, but right now it's on my other car. If I take it off then the other car isn't drivable.

So there is no good way to test the Maste cylinder other than process of elimination??

[Bennetts22]

otherwise i think i have a master laying around. do you want me to send it to you dave?
let me know. Brandon