I have an 86 F-250 6.9 diesel. I had brake issues. I replaced the brake booster, master cylinder and one wheel cylinder. Then flushed and bled the brakes. The brake pedal still feels a but "mushy" and the brakes work fairly well when first applied, but then fade as you hold to stop, or if you let off of the brake and re-apply them there is very little braking power. Can this be the booster not getting proper vacuum? How do I test for proper vacuum?

Can someone answer this?

 

This is the wrong forum for your truck, but you may check the check valve on your power booster. They are cheap and if you did not replace yours, it may give the problems you are reporting.

 

Where is the booster getting vacuum from on your motor? Are you losing any brake fluid? When was the last time the rubber brake lines were replaced?

 

He has a 86, so this is the right forum. If the booster is not getting enough vacuum, the pedal will be high and very hard to push.

If your brakes a low with lots of pedal travel, I would suspect there is still air in the lines or the rear brakes are not adjusted correctly.

The first easy test, push the parking brake pedal down about half or 3/4 the way down, and then try the brakes. If they pedal is a lot higher, and the brakes seem ok, then you need to go back and adjust the rear brakes.

Release the parking brake(if it will release, these things like to rust and stick). Go back and jack the truck up, turn the wheel by hand while turning the adjuster. Turn it till it locks the wheel, and then turn it to release the adjustment till you can turn the wheel, but there is just a little rubbing. Do both sides.

If after this, your pedal is higher but it's still mushy, you still have air in the lines. I know from working on these trucks that the rear system holds more fluid than the reservoir in the master cylinder can supply while bleeding(the little one goes to the rear). If you didn't bleed it till it was about half way down in the reservoir, and then refill it and bleed it till it's about half way again, then you didn't bleed the rear enough. It takes about the whole capacity to bleed the rear lines enough.