Master cylinder or booster?
 

OK, I am still uncertain as to which one to replace. Does anyone know how to perform a test to isolate the two? Here is my problem;

No pedal, even after bleeding. The bleeding process seems good, until I start the truck and I loose all pressure instantly. The rears are adjusted good and I have plenty of pad in the front. I need to determin which one to replace. I can keep pumping the pedal until I get "some" pressure, but when I keep pressure on the pedal, it goes back down to the floor. I am working with a 1995 Bronco. Also, no visible leaks and the problem came on suddenly.

Any and all help is appreciated,

TK

With the truck off, pump the break pedal. Does it get hard and high and stay there until the truck is started? If it does and then falls to the floor when started its not the booster. It almost sounds like you lost a seal in the master allowing a severe bypass condition.

OK, I changed out the master cylinder and still no pedal. Actually, it is a little better, so I guess it did not hurt to change the master cylinder. Now, I have great pedal pressure when I pump the brakes with the motor off and the pedal stays pressurized. But, as soon as I start the motor, the pedal goes down to the floor. One thing I did not do, and I am kicking myslef now, is measure the push rod distance. I guess I figured that I did not disturb that part, so it should not have needed adjusted. I can take the MC back off tomorrow and check the diatance without effecting the lines. This will prevent me from bleeding AGAIN!!!!

So, any other suggestions? I am open to all.

Thanks,

TK

A good booster will get high/hard with the truck off and then when started it should go down showing it evacuated the vacuum helping you apply brakes. Is your vaccuumn system up to par? The pushrod shouldn't ever have been changed so there is no reason to adjust it.

Try bleeding the brakes with the truck running, it sounds like you still have air in the lines. Just do not go crazy pressing the pedal until the pedal is firm or you could pop a line.

Update:

Not being totally satisfied with the "self adjusters", I removed the drums to find them both bad and almost no linings on the shoes. Duh, I thought!!! Well, after replacing the drums, shoes, adjusting and re-bleeding...I still have no pedal!! Obviously, I have better brake action now. I am about out of ideas. I hope you guys have more coming. Also, I put the entire truck up in the air and locked in the front hubs. When I activated the brakes, all four wheels stopped really good.

TK

Did you check the vaccuum? When you say no pedal do you mean you have to hit the floorboard before you start breaking?

The only way I have to check vaccuum is to remove the hose with the engine running. I did that and it seemed to have pretty strong vaccuum. As I pump the brakes, the pedal gets high and hard. While keeping pressure on the pedal, I start the truck. As soon as the truck starts, the pedal creeps down towards the floor.

TK

That should happen, that indicates that the booster is functioning. As for the pedal being soft/not there, do you hit the floor before you start breaking or it just isnt as high as you want? After a break job the brakes wil lbe soft until the pads/shoes wear into the rotors/drums. The only thing that makes a soft/spongy pedal is air in the system. What year is the truck? ABS or no?

If you didn't bleed the ABS valve, that's most likely the problem.

How do I go about bleeding the ABS valve? Do you have a website that will point me in the right direction?

TK

Most people will say that you have to have it professionally powerbled to get the air out. I have found if you have everything adjusted and bled as good as you can get then take it out on a back road and brake HARD so the ABS kicks in. Do this a few times and then double check your lines with a quick bleed and you should be good to go. Has worked everytime for me, if you don't want the hassel take it somewhere and ask for a pressurized brake fluid flush, will do the same thing and completely replace old fluid.