brakes bleeds down

i have a 92 ford f250 deisel.....and we replaced the master cylinder rear wheel cylinder and calipers...bleed the system manually and with a brake flush machine...and still bleeds down when vehicle is turned on???whatelse could it be?? booster? vacumm pump???

Is the power assist vacuum or hydro-boost? If it is vacuum assisted, a bad pump will give a harder than normal pedal. If it is hydro-boost, check fluid level. When you replaced the master cylinder, did you replace it with the same size bore in the master cylinder? I have replaced m/c and found out the bore was smaller and caused bleed off.

Is the power assist vacuum or hydro-boost? If it is vacuum assisted, a bad pump will give a harder than normal pedal. If it is hydro-boost, check fluid level. When you replaced the master cylinder, did you replace it with the same size bore in the master cylinder? I have replaced m/c and found out the bore was smaller and caused bleed off.


i got a new master cylinder cause the last one was leaking behind master cylinder.....but when i benched bleed it...it got very hard...so i let sit over nite and try again in the morning....but TY for your advice....if u find out anymore then let me know..i will check back in the afternoon...ty