Pickup truck brake problems

Question:

I have a 1977 Dodge Warlock ¥-ton two-wheel drive pickup truck. Twelve years ago I put on all new brake pads, rotors, calipers, master cylinder, rear brakes and then I bled the brakes four times.

When the truck is not running I have a nice, high normal brake pedal, but as soon as I start the truck and it gets vacuum from the brake booster, the pedal goes down to one inch from the floorboard. The brakes stop the truck just fine, but it is kind of scary for the pedal to go down that far.

I drove the truck that way for 12 years, but in July of 2014 I had my mechanic install new brake pads, rotors, calipers, front brake hoses, master cylinder, rear brake shoes, wheel cylinders, rear drums,

brake lines, etc. so everything is new. The mechanic bled the brakes three times, and the brake pedal still goes down almost to the floor. I checked all of the fittings for leaks, and everything is in good working order. Also, when the truck is running, the vacuum reads between 12 and 16 inches of mercury, What could the problem be?

Answer:

I would begin by making sure you \ have the correct master cylinder for your truck. If the check valves in it are not right for disc and drum brakes that could be your problem. It is possible that the wrong master cylinder was installed 12 years ago, and the new mechanic just replaced it with another one of the same kind. Make sure it is the correct part number for your particular truck and configuration, and consult a shop manual to see how it should be hooked up.

Also, did your mechanic bench bleed the master cylinder? Brakes can be very hard to bleed if there is air in the master cylinder. I would also make sure you have a proportioning valve installed and that it is plumbed correctly according to the shop manual too.

Ido not believe your brake booster is the problem because usually when that fails, the brakes are very difficult to apply, but the pedal does not drop to the floor.