Ok i have a problem that i cant seem to figure out. In my 78 f250 i replaced the brake booster, master cylinder, brake pads, and calipers. The problem i have is when i use the breake alot pressure builds up and doesnt get released and the brakes drag and causes my truck to really work hard. Now i know about the adjust thing on the end of the brake booster but the thing is i have it adjusted so much when i first start the truck and use the brakes you have to put the pedal to the floor to just barley stop the truck, but as i use the brakes pressure will build up and then i can stop a lot faster, but if used to much they will drag. If they drag and i will pull onto the side of the road and loosen the master cylinder and you can hear pressure release then the brakes are fine until i use them to much and drag again. The brake hoses seem to be torn up a little and i was told by someone that after time the brake lines will swell and collapse and maybe that happened because i dont know how old the hoses are, and then my dad said it could be the porportioaning valve not working and releasing the pressure. What do you guys think the problem is. Also when i bought the truck it didnt have a brake booster or mc so the brake havent been used for a long time and brown brake fluid came out when bleeding the lines.

 

Hi. It really sounds like the pedal push rod or something else is not letting the master cyl. piston return all the way. That piston has got to return all the way or it will do just what you said it's doing.

 

Was it doing this before you replaced the booster, master and calipers? Some 73-79s have a bracket between the booster and firewall with linkages and a pivot - does yours? If it does, make sure this pivot and linkages are moving freely - some have grease zerks for lubing. Could be that rusted linkages are keeping pressure on the brakes.

Do a search, there is measurement for proper adjustment of the rod coming out of the booster.

 

I read your post again and I once found a hose with the inside collapsed so bad it would let a little pressure to the brake but none would return to the master cyl. That one had a definate one wheel lock up smoke and all. The thing was much older than yours. I also have never seen a proportioning or combination valve show the symptoms your having. Make sure the master cyl piston is returning all the way to the snap ring or what ever stops it from coming all the way out of the cyl.