beater

Well after basically redoing the front brake system including<master cylinder,power brake booster,calipers, and new brake lines, my brakes pump up and I can feel air in the lines but brake pedal holds fine, as soon as I start the truck and drive it 30 ft the brake basically goes to the floor with little to no effort and honestly the brakes dont stop good at all. I bypassed the rwal valve and still the same thing. Do you guys have any ideas what it could be? I know my back brakes are working somewhat so I am at a lose of what it could be.

TIMMY22

how much vacuum do you have?

Chrisreyn

My guess would be that you
1) dont have them bled completely
2) dont have the MC bled properly
3) have a vacuum issue- its easy to breakthe seal of the vacuum hose going into the vaccum assist while changing the master cylinder, oreven to break off one of the hose fittings on it.. check that

beater

I bench bleed the master properly, the attachment for the vacuum assist is fine and brand new. I took the vacuum hose while truck was running and it seemed to have fairly strong vacuum, I have no gauge to verify it but I do believe to be enough. Would air in the lines cause the pedal to go away like that? Ive had plenty of cars that had air in the lines and none of them had the pedal basically go away. I can pump up the pedall while moving and get some pedal pressure but it still doesnt seem to want to stop the truck to good even when it is pumped up.

dzl_damon

I"m gonna vote bad bled MC. Driving down a bumpy road after I tried bleeding mine resulted in if feeling better. It was acting quite strange before it shook the air bubbles out of it.

Second MC I did, I did the bench method of bleeding. Works better since it's level instead of a slight rake.

EDIT: just saw your post you were writting at the same time... My dually i instinctivley give a few pumps before a known stop sign cause the brakes work so much better. I'm told this is a faulty MC. And if it's a Napa rebuild, I would definately NOT rule out a bad rebuild! I had one and have read of several more.

beater

I guess I will pull of the MC tomorrow to rebleed it, I got it from advance auto parts so it probably isnt a very good one to begin with. I bled it for I know 75 pumps lol, and even when bubbles quit coming out I pumped it a few more times just to make sure. Thanks for the help fellows.

maybe368

If I were you, I would borrow or buy a vacuum gauge. You can get a cheap mity vac from harbor freight or any discount auto parts store for around 10 or 15 bucks. I have found that the vacuum needs to be much better than the 8 inches Hg that is the minimum listed in the FSM. 18 was not enough on mine. Also. I believe that the rod that actuates the MC has to be adjusted for each MC, so that it is actually better to buy a paired set of booster and MC, because they are adjusted at the factory. I went through every problem of these brakes and have found that EVERY part has to be working correctly and be adjusted correctly. The are exceptionally finicky, Like a Ferrari Mark

maybe368

Here is a scan of the adjustment instructions that came with a MC,,,Mark

Jim5870

Keep you old MC cover and put in air line fitting, and then you can use it
as a pressure bleeding tool using compressed air set to about 20 psi.

maybe368

I believe that you would also have to open the valve on the front of the proportioning valve...Mark

beater

pulled the hose off the powerbrake booster and it held steady at about 28 psi so that can't be an issue I also checked over the master and my actuator rod is set perfect according to 2 different specs only thing I think it could be was air in the lines still.

93 12v

I recently put 1 ton wheel cylinders on my truck, and was doing the same thing. Could pump them and get good pedal, but after letting off and trying again 30 nft later the pedal would go to the floor and lights would come on. I rebled the rears and all has been fine, except I have to replace my MC again (been leaking down awhile).

I vote air. You might try bleeding until you get fresh fluid at all 4 wheels, because moisture really screws up brake fluid's effectiveness too.

beater

anybody know the size of the bolt that holds the back rotors on?

beater

anybody know the size of the bolt that holds the back rotors on?

TIMMY22

Back Rotors?