Hi again everyone, I am onto my next problem in the quest to make my car a reliable driver, when I purchased it about 6 months ago, it had reasonable brakes, it had stock front discs and calipers, a generic rear disc conversion, a stock master cyl on the firewall with a remote booster setup mounted up front opposite side to the battery, with no boosting to the rear and a willwood manual valve to the rears, I had a leak at the remote cyl on the booster so I decided to put a normal booster on the firewall and get rid of the remote one, I bought a 8" dual diaphram booster(one of the chrome ones) and a matching master cyl in 1 1/8 bore. For the life of me now I can't get this thing to stop, I push hard on the pedal and it feels like I have a brick underneath it, but it just slowly comes to a stop, I still have the willwood valve to the rear, I can wind it all the way in or out and it doesn't make any difference, I am not running any other proportioning valve.
Any ideas?

I am getting told by a few guys over here I should have fitted a 7/8 bore master cyl as it would give it more brake pressure, I don't want to buy another one unless I could be sure it would fix it.

 

Sounds like a vacuum issue, and the booster isn't functioning. Have you put a vacuum gauge on the spply line to see if it is getting enough vacuum? I bet if you disconnected the vacuum line the brakes would feel just like they do now.
Get a T to go in the feed line, and check vacuum to the booster.

 

Funny you should say that, I thought the same, but everyone keeps pushing me in different directions with it. Any idea how much vacuum these things need to work properly?
And if that is the problem, how do I get more vacuum?

 

Best is 15 inches or more, but they will work with 12 inches. Anything less will not work well. Only way to get more vacuum is a different cam, or a vacuum pump to replace the engine vacuum system.

 

I just got the engine running again today after a dizzy rebuild, I put a vacuum gauge on the line out of the inlet manifold and at idle I am only getting about 8-10in of vacuum, if I hold it at 2000rpm it goes up to a bit over 15in, I would say that is my problem. I just have to work out what to do to fix it?

 

Has anyone tried the one valve into the booster and/or a vacuum tank?

 

Not sure what you mean by "the one valve"? Be sure you've got the brake booster connected to the full manifold vacuum port on the base of the carb, with nothing else hooked to that point. You don't want it hooked to timed vacuum.

 

No I have the vacuum coming directly out of the manifold on its own, I don't have a vacuum advance, am running the MSD Billet Distributor setup, the one way valve I meant is Aeroflow Billet Brake Booster ONE WAY 3 8"Barb NON Return Valve Vacuum Race Drag | eBay

 

"One way valve" makes much more sense! Back check valves might help, but I've never seen a reason to use one on a vacuum system. On a pressure system they will keep the pressure up, and not allow it to flow back towards the supply.

 

Having a check valve in the brake booster hose isn't all that uncommon. I know I've had them in some of my cars. What it will do is hold vacuum in the brake booster should your engine stall or if you get a vacuum leak. That stored vacuum will give you one maybe two pumps of the brakes before the pedal turns to stone.