First off Im a competent mechanic or I believed so before this. Had weak front brakes for last 2 years and noisy rears so I replaced rear pads turned drums new calipers up front and bled all wheels repeatedly no air. have a very low pedal asif i have no pressure to front calipers but get plenty of fluid from front calipers. brakes pump up then i start truck and im back to no pedal pressure I test drove truck and rears will lock up if crammed to floor no front brakes . replaced master cyl. yesterday assuming was faulty same thing went to salvage yard got a combi and proportioning valve same thing im stumped all i have left is a booster I tried my backup booster (one I had in the garage) works exactly the same this is my truck i have 400.00 in brakes with no fixes as of yet and i love this truck but its gonna brake me soon if i dont get this figured out.....plz help

 

I stated rear pads but they are rear shoes sorry.

 

Get someone behind the brake pedal to mash down while you carefully inspect all of the soft lines in the system. Every time I've seen the problem you describe I've found one or more lines not just swelling, but ballooning. If you find one, replace 'em all because you'll end up doing it one at a time anyway.

 

Brake lite in the dash on?

 

I am thinking the hoses collapsed, but just guessing.

 

When you did the bleeding, did it behave as expected? Like it was even with the pedal pressure or however method you used?

Just curious as I'm not sure it will tell you anything.