Power brake booster

[dans55]

Can someone tell me what constitutes a defective power brake booster? I added disc brakes to the front and with drums on rear. The booster,m/c (Corvette style) and disc/drum prop. valve are all new. All lines have been bled more than once.Didnt buy these as a kit. With my car running,the pedal goes 2 inches from the floor before any sign of braking. After I stop the motor,I have about 1/2 pedal. If I disconnect the check valve at the booster to release vaccuum,which by the way is 15 in. at the valve, the pedal goes to the floor. I have replaced the prop. valve and m/c again with same results.Can the booster be bad? any suggestions Please.

[chevynut]

I would think it would be HARDER to push the pedal down if you had a bad booster. Not sure why you would only have half pedal, with the engine off, but the pedal goes to the floor with the vacuum disconnected. Seems like it should behave the same.

I would check to see if the booster pushrod is going into the MC correctly...it may be too short. Measure the length of the pushrod protrusion out of the booster, and the depth of the hole in the MC. Are you getting any fluid at all when you push the pedal to bleed it?

[chevynut]

Read this:

http://www.classicperform.com/TechBo...oubleshoot.htm

Check Booster Pushrod Adjustment

The pushrod that actuates the master cylinder must be properly adjusted. Ideally there should be only slight clearance between the booster pushrod and the master cylinder pushrod. Interference will preload the master cylinder. When the system is preloaded, it builds pressure each time the pedal is pressed. Since the master cylinder is not allowed to fully release the pressure from the previous stroke, the system will eventually lock the wheels. Too much clearance will cause excessive freeplay in the pedal. In almost every case, if you bought your booster and master cylinder from us, we pre-adjust the booster pushrod and master cylinder for you so this is not a problem.
To make 100% certain of a properly adjusted booster pushrod for your master, we offer a master cylinder depth gauge (PN: CP2003) to determine correct pushrod length. The following illustration demonstrates the proper usage of this tool. First measure the length that the booster pushrod extends past the face of the booster with the "head" of the pin. on the tool. Then measure the depth of the master cylinder piston with the other end of the pin on the tool. If the pin hits or there is too much space on step two, then adjustment is necessary.
There are basically two different master cylinders that we use for our Chevy and Ford kits, the Deep Bore master cylinder and the Shallow Bore master cylinder. To determine which master cylinder your booster is setup to mate with, you can do the following simple check. For Shallow Bore master cylinders, the pushrod should be approximately flush with or below the booster face. Deep Bore master cylinders will protrude past the face of the booster body by about 1" to 1-1/2". Note that Deep Bore master cylinders are required for manual brakes and Shallow Bore master cylinders are generally used for power brake systems. We now supply a master cylinder piston adapter to accommodate both master cylinders. You will only use this adapter when you have a Deep Bore master cylinder with a short booster pushrod.

[dans55]

I would think I would have a hard pedal without vacuum.not the case.I am getting fluid at all wheel cylinders.I disconnected the line for the rear brakes from the prop. valve and got fluid as soon as I push the pedal.

[chevynut]

So you don't ever have a hard pedal? Sounds like you said you get 1/2 pedal with the engine off. I interpret that to mean you get brakes without vacuum.

If you get fluid immediately when you touch the pedal, the booster pushrod is probably okay. IMO I'd check it anyhow to be sure it's adjusted correctly.

There's something wrong with your system and I don't think it has anything to do with the booster unless it's a pushrod adjustment issue. All the booster does is help you push the pedal down with vacuum on one side of the diaphragm. What size bore does the MC have? What disc calipers are you using?Does the master cylinder have residual valves for the rear drum brakes? Have you adjusted the rear brakes correctly? Lots of things to check.

Check the things in the CPP list and I'll bet you find the problem.

[Rick_L]

CPP has a good booster troubleshooting article on their website, as does mpbrakes.com.

Some movement of pedal height is normal when comparing between engine off and engine running. But the booster doesn't allow the pedal to go to the floor when it goes bad. When the booster is bad, at worst you have no boost. That's a marginal situation on many vehicles though, as master cylinder bore size and pedal ratio are done so that they partially counteract boost - but that's strictly how hard you have to apply the pedal, not whether you have a pedal at all.

[dans55]

Thanks Chevynut. Maybe i can figure it out.

[dans55]

Rick_L. I do have some braking at about 2 inches from the floor. If I had to stop quickly, forget it.I have bled and bled the brakes starting at the rear and have 'airless' fluid on all 4 corners.I just dont have a good pedal. The play in pedal travel from the stop till I feel resistance is approx. 3/4 to 1 inch. Speaking of the booster,if there is no vacuum present shouldnt there be a 'hard' pedal?

[chevynut]

Based on what you're describing, I'm betting your master cylinder and calipers are mismatched, and perhaps you don't have residual valves in the rear drum brakes.

[dans55]

I thought the residual valve was built into the prop. valve.