I have replaced front calipers and hoses, rear brake shoes and cylinders, turned rear drums, new master cylinder. I have manually bled all air out of system(about 10 times so far). Each time I bleed it, I get more air. There are no leaks detected anywhere in system. The truck is a f350 with the 7.3 diesel motor. It has vacuum assisted power brakes. When pushing the pedal, it is very low and spongy and will slowly go to floor. Before I start the motor, the pedal is somewhat hard but after starting, it will go to floor. It has little to no braking(unsafe to drive). The vacuum pump is in good shape and provides good vacuum to booster. The booster is in good shape, too. Truck has the rear antilock system(no warning light is on). What gives with this truck? I have been working on it for a month and had 3 local mechanics assist me, all are stumped.

 

ok have had same problem with another vehicle you could have a leak getting air in and not fluid out bleed at master cylinder but what i had to do on this vehicle is block of one wheel at a time or you can block off the front and then back at eh master cylinder or the poportioning block until you get a pedal and then you will know which wheel or line,hose etc. whatever is causing problem could be threads around line try thread tape or same with bleeder screw, hope this method helps.

 

Did you ever figure it out? I have a 1995 F350 7.3l and have replaced EVERYTHING and still same problem. Soft pedal that leaks to the floor. I have put over $1600 in the brake system and nothing has changed. I have also taken it to a shop and have contacted the Ford dealer. I have had the MC replaced with 4 different ones (last one is from Ford).. I am at a loss and am about to sell the truck if I can not figure it out.

 

When everything else failed, I went to Ford dealer, told them my problem(soft, spongy, and little or no brake), they showed me a bypass fitting they use to disconnect the brake line in and out on the ABS hydraulic unit, located inside frame on left near front. Connect the 2 lines together, bleed system again, and try brakes. If this solves problem, the ABS hydraulic unit is bad.

 

I changed everything twice. Fail. It wound up being loose front wheel bearings causing the rotors to wobble and spread the callapers. 1 turn out of adjustment is all it takes. Pedal sinking is normal on 4 wheel disk systems.

 

brakes are spongy,calipers are good, pads are good,no leaks, booster working fine, when brakes are applied pedal slowly goes down, if applied to hard right front tire locks up, i'm confused

 

1991 f350 diesel. Replaced front pads and calipers and master cylinder. Brake seems fine on initial pressing but subsequent pressings in quick sucession gives a very very hard peddle with virtually no stopping. I am really confused. Can anybody help??

 

There is a condition that applies to worn rotors and that is called taper. When you put new pads up against tapered rotors you don't get full pad contact and that can make stopping somewhat difficult since only part of the pad is actually contacting the rotor.
If you did not check the rotors for taper you really did only 1/2 of a good pad replacement job. If you had good brakes before replacing the pads and calipers that could possibly be the problem because the old pads could show more wear on one part of the pad compared to another part of the same pad.
Also . the vacuum pump may be on it's way out since it can't keep up with the vacuum requirement of repeated brake applications.