I put a new mastercylinder on my 91 gmc but the pedal goes down, i can pump the pedal up but then it still creeps down when holding pressure any sugestions?
hate to say this, is this a defective unit? no old brake fluid? something rotten in denmark.try a good master cylinder bleed, just thought of this and maybe you could try this....try opening up the slave cylinder and using a shop vac to remove the fluid...wild idea..eh? but that won't help you to evacuate ALL the bubbles if you can do it. perhaps one mfr makes a different color than coffee honey, be nice to witness your progress, with your friend activating the bleeder. Perhaps your helper is just not being aggressive enough and you guys are not bleeding it properly...try again please, this time with a little gusto and some fresh..I mean fresh brake fluid, would be my recommendation.
if its creeping its leaking somewhere ,if you did that long enough you;d see fluid, check your connections,sometime a bad new part also