Power Brakes and Paxton

[texas swede]

Hi guys,
Need your help as usual. A friend of mine in Sweden, former NHL player Daniel Tjernqvist, owns a 67
Shelby, #153 equipped with a Paxton. He still has problems with his brakes. No power assistance.
The booster has been replaced to no avail. Vacuum at idle is 12-13 inches of Mercury which seems too low to me.
My 67 with a HiPo makes 15-16 and brakes works fine. A race shop owner in Sweden just looked at the car by opening the hood
and said, the Paxton is the problem as it creates pressure and the booster works on vacuum.
Is he correct? Many 67's are equipped with Paxtons and I wonder if they have the same problem.
Any expert ideas?
thanks,
Bo

[PR]

I have power brakes and a Paxton with no problems, I had my booster rebuilt awhile back, brake pedal was hard but brakes sucked, I would take another look at the booster and check valve,

[roddster]

  When you get off the throttle to push the brake pedal, the throttle plates close and up comes the vaccuum.  Something is up with the booster, lines or check valve.

[texas swede]

The booster was repaired and another Swedish friend took it apart and everything was like new and no problems with
it. Check valve is also new. Will ask him to check the hose. What about the low vacuum at the intake.
Texas Swede

[JD]

The guy to talk to about it is Craig Conley.
http://paradisewheels.biz/power/

[shelbydoug]

The booster needs to be verified that it holds vacuum. It seems a simple rebuild but because it is simple, it is tricky. Trust me. That sounds like it makes no sense whatsoever but it's an issue. I screwed up my rebuild and had to send it out to a rebuilder.

With a stock HP cam, vacuum at idle should be higher. 14-15 is what you want. To me 12 indicates a vacuum leak and probably through the power booster. That's why you can't hear it. It's internal.

Frankly, under 15 inches the boost is going to be marginal and the brakes will probably feel a little hard.

I think the check valve is what keeps the booster working under boost. I can tell you that on my Audi TT it's turbocharged and blows 28psi. It's got no problem with accessories loosing vacuum boost.

A Paxton by comparison at 6 psi is child's play. There should be no issues caused by the blower. You may need to change the check valve to a higher pressure rating though.

[Sixx7shelby]

No problems at all with brakes and a Paxton for me.

[texas swede]

Thanks ya'll as we say in Texas,
I got some more information from Roland in Sweden today. He is the one who replaced the booster
with a rebuilt one I brought over from Texas. Roland says the brakes worked fine when Daniel
took a test drive but when the car got warm they stopped working again. Looks to me it can be vacuum leak at the
manifold which drops the vacuum. I asked Roland to help Daniel by re-torqueing the intake manifold bolts.
Do you agree that this may be the problem. If it's something internal in the booster like a check valve etc. it should have the
same problems when engine is cold, correct?
Texas Swede 

[shelbydoug]

Thanks ya'll as we say in Texas,
I got some more information from Roland in Sweden today. He is the one who replaced the booster
with a rebuilt one I brought over from Texas. Roland says the brakes worked fine when Daniel
took a test drive but when the car got warm they stopped working again. Looks to me it can be vacuum leak at the
manifold which drops the vacuum. I asked Roland to help Daniel by re-torqueing the intake manifold bolts.
Do you agree that this may be the problem. If it's something internal in the booster like a check valve etc. it should have the
same problems when engine is cold, correct?
Texas Swede

I think that if you had an obvious manifold leak you would have a backfire. These cars are over 50 years old now. Strange things can happen. For instance,  the intake manifold could have rotted through the exhaust crossover. That's more common on the FE's because of the pipe plugs in them but I've chased gremlins also. Some are affraid of me and hide only when I am looking for them. Then come out when I go away.

[1967 eight barrel]

I have spoken to Steve at Power Brake Exchange about this issue before. He stated that they can work them to make them more sensitive in low vacuum situations, but at the end of the day they need at least 15 inches of manifold vacuum to function.

                                                                  -Keith