93 F-250 Brake Problem

[IDIPWR]

I have a problem with my 93 where I will loose what seems to be the (very hard brake pedal) power assist to the brakes. I woll have normal brakes about 75% of the time and no power the rest. I have changed the vacuum pump with no change. What hat could the problem be? The booster? Any ideas? Is there a check valve anywhere that could cause this prob? Also what is the thing that has wires hooked to it that also vas a vacuum line to it from the vacuum junction block on the drivers innerfender?

[blageurt]

Either the booster is going south or you have an internal leak in the master cylinder. Disconnect the vacuum to the booster and see if you have brakes. If the brakes work normally except for them being hard, its the booster. If there is no change it's the master.
Or , somehow air has entered the system and you have to flush /bleed the entire system.
When is the last time you flushed the fluid. If it's more than 2 years you have to change the fluid completely.

[cdnsarguy]

The Tuna can on the fender is the low vacuum sensor.....

The check valve on the booster has been known to cause problems..... replacement should be inexpensive....

Do the vacuum test on my photo link.......