[RatPin]

So I finally started and drove my frame up build yesterday. One of the big issues I'm having is no brake pressure. I am using a new frame mounted MS with 7" booster, vacuum port to carb. I just re-bled the system and no air. The wierd thing is I have decent pedal until I start the engine then it goes to the floor while running. Shut it off to decent pressure again, not good but decent. Any help?

[LATECH]

The pedal "feels" better with the engine off as there is no power assist, making the pedal hard to push. Starting the engine make the booster multiply force,revealing then ...that you have no brake pressure.
A few things here...
Is it a disc /drum setup?
Disc /Disc ?
Anytime a master cylinder is mounted low on the frame, residual pressure valves need to be installed .
A residual pressure valve holds the wheel cylinders/ pistons in calipers closer to the friction area of the drum/rotor. Taking up any slack, making the pedal travel required to activate the brakes considerably less.
Another thing to consider is the bore size of the master cylinder, if the bore is too small, it wont be sufficient to move the disc brake pistons out to begin with.
More info on your system is needed.

[RatPin]

Thanks for the reply. It is a disc/drum and I bought the residual valves but didnt install them because two experienced local builders told me with the GM master cylinder I did not need them, like the residuals were built into the GM cylinder. Is that not actually the case?

[LATECH]

Some GM masters did not have them. You need to find out if yours has them. Chances are it does...in the rear....but maybe not the front .
That needs some research...as you will want 10 PSI valves in the front an 2 PSI in the rear.
What bore size is the master and does it have a big and small reservoir?

[enjenjo]

Originally Posted by LATECH

Some GM masters did not have them. You need to find out if yours has them. Chances are it does...in the rear....but maybe not the front .
That needs some research...as you will want 10 PSI valves in the front an 2 PSI in the rear.
What bore size is the master and does it have a big and small reservoir?

That would be 10 PSI with drum, 2 PSI with disc. The Brake Man - ValveTechI

[RatPin]

I'm using kit numer BR2918 from american classic.

Here it is:

American Classic Truck Parts Inc.

Does that tell you anything? I really appreciated.

[aosborn]

That system utilizes a combination valve. No other additional valves need to be added. Just make sure you have the lines connected to the proper ports on the combi valve. Typically there are two ports at the front of the valve for the front brakes, and one out the rear for both rear brakes.

Make sure the pushrod between the booster and master cylinder is adjusted properly with minimal clearance, and that the pedal is installed with enough travel to bottom out the master cylinder.

Does the pedal pump up indicating that there is still air in the system?

Regards,

Andy

[RatPin]

I had to shorten the threaded shaft that comes out of the booster so it would extend out all the way when pedal is all the way out. It travels a long way so I would think it completes it's stroke. Pressure does not build when pleseing pedal.

[aosborn]

So the pushrod to the pedal sounds like it is adjusted properly. Next, you need to check the pushrod between the booster and the master cylinder.

Remove the master cylinder (you don't usually need to remove the brake lines) and check to see if the small pushrod coming out of the booster is threaded and adjustable. If it is, lengthen it 2 or 3 turns and then slip the master cylinder back into position and see if you can feel the pushrod touching the master cylinder. If you can, shorten the pushrod 1 turn and try again. Just when you get to the point where the master cylinder will mate back up with the booster without touching the pushrod, shorten it 1/2 turn more and you are done. If it is a little out of adjustment it makes a big difference in pedal travel.

[RatPin]

I will follow the steps above today to see if I can solve the problem and report back. There is only one line going out to supply both front discs.

[RatPin]

Ok, I extended the shaft between MS and boster about 2.5 turns out and it did give me a nice firm pedal and brakes grabbed well. I took the truck up and down the driveway and while turning around and holding pedal firm the pressure seemed to slowly fade and go away. Do you think I need to re bleed the system now since making that adjustment? Or should I extend that shaft out another half turn? The important part is we are making progress here!

[LATECH]

If the pedal was falling away from your foot towards the floor (pedal drop) , then you either have a leak, or the master is bad.

[RatPin]

After closer investigation I found a banjo on a front caliper loose with a slow leak so that would explain the slow pressure loss. I'll re bleed the front reservoir and try again.

[RatPin]

Got it figured out for the time being. Thanks for the help!

Here she is...

[RatPin]

Man I'm really having trouble getting these brakes dialed in! I drove it to work this week and after driving about 20 minutes pressure builds up and the brakes drag. Shut it off for a few minutes and it goes away for awhile only to come back. I have adjusted both rods all over the place and I either get pedal to the floor or full pedal stroke before solid pressure right at the end of the stroke, but that's when the pressure builds and causes dragging.

Am I supposed to have a firm brake pedal right at the top of the stroke? With mine I have about 80% pedal stroke with what feels like light vacuum assisted braking, but the pedal really does not get firm and stop until the very end of the stroke and thats when I get the pressure build up. If I adjust the rod ine turn off then the pedal gies to the floor without any stopping pressure.

My pedal ratio is 6:1 and have a vacuum line that is about 30" long from the floor mounted booster to the carb.

Any insight is help is very much appreciated guys. I dont want to buy a new master if it might be something else. I just want to drive my truck?