hey, just recently bought an 84 f150, and im having a little trouble with the brakes. I went ahead and put on new front calipers, and lines from the front to back. they all seemed to have bled properly, however the pedal goes pretty low.

when I took it for a test drive, even when the brakes go right down, the truck will still stop fine, so I know there is no air in the lines. when I have the truck reved up a bit while im driving, the brake pedal goes right back up and feels pretty good, but as soon as the RPM's drop the pedal drops aswell.

im thinking this is either the booster or the proportioning valve. booster because like I said when the trucks rpms are high, the vacuum would be high, which would increase pedal. reason I think maybe the proportioning valve is because my old Honda civic did similar, and that's what it ended up being.

just thought id get a couple opinions before I go out and spend money I don't have much of right now haha.

 

Worst case scenario fix the cheaper one first. The brake booster will set you back about $100, the proportioning valve is closer to $300. If it's not the booster then at least when you replace the valve your whole brake system is new.

 

It's not the booster. The booster only gives vacuum to give you power brakes. If it goes bad the brake pedal gets hard. I had a very similar issue. I replaced absolutely everything, The booster, booster vacuum hose, all the lines, brake pads, rotors, and even replaced the master cylinder twice. Only thing I did not replace was the proportioning valve. I tried gravity bleeding and nothing. Rented a pump to bleed and nothing. Haynes tells you if you get a leak the proportioning valve moves to one side, and then it tells you how to reset it. The reason I had replaced the lines was because the previous owner had blown a back line, so I followed the instructions to a tee several times to reset it. I literally worked on this for 4 months. In the end, some jerk cut me off, and I slammed the brakes on and it fixed it. So my advise is go to a back road, go up to about 15 to 20 mph, and slam the brakes on as if someone cut you off. It worked for me.