[kbuhagiar]

Hello folks,

Just purchased a 1947 Plymouth street rod, and I am in the process of assessing its needs.
One of the things on my list for improvement is the brakes, or rather the power assist (or lack thereof).

The car has a Nova front end with disk brakes, and a Nova rear with drums. The previous owner had an aftermarket power brake MC unit installed (under the floorboard in the factory location). I haven't had a chance to climb under the car to do an inspection, so I don't know the manufacturer or part number. I do know that an aftermarket mounting brace was added under the car, either to supplement the existing factory mounting or to create a new mounting location to facilitate the aftermarket power brake MC.

The brakes seem to work well, but there appears to be no power assist whatsoever. The vacuum assist unit is definitely hooked into manifold vacuum, as you can feel and hear the engine react when you press the pedal. In fact, after a long brake application, the engine (1987 Corvette 350 TPI) stumbles significantly, as if sacrificing great gobs of vacuum. The engine idles fine otherwise, although I have yet to check it at idle with a vacuum gauge

Any ideas as to what may be happening here? My first instinct is to suspect a problem with either the booster or the check valve. I have heard stories of problems with pedal geometry creating extra effort, but only with firewall-mounted units.

Thanks in advance for your thoughts and ideas.

Cheers, Ken

[OneMoreTime]

I would check for a vac leak first as those usually work well..could be in the line from the engine to the booster..maybe as simple as a loose connection or maybe the vac line has a hole in it or it has contacted a header..

Sam

[sunsetdart]

Because it has disc/drum there should be a 10lb residual valve in the rear line and a 2lb valve in the front. If they are not there, then you may have a problem down the road with a very spongy pedal if you have a pedal at all.

[Rickracer]

Sounds like probably a bad booster,

[cobalt327]

Originally Posted by kbuhagiar

The brakes seem to work well, but there appears to be no power assist whatsoever. The vacuum assist unit is definitely hooked into manifold vacuum, as you can feel and hear the engine react when you press the pedal. In fact, after a long brake application, the engine (1987 Corvette 350 TPI) stumbles significantly, as if sacrificing great gobs of vacuum. The engine idles fine otherwise, although I have yet to check it at idle with a vacuum gauge

Any ideas as to what may be happening here? My first instinct is to suspect a problem with either the booster or the check valve. I have heard stories of problems with pedal geometry creating extra effort, but only with firewall-mounted units.

Along w/problems w/the booster or check valve, you can add mismatched master cylinder piston diameter to the pedal geometry.

[adantessr]

The brakes seem to work well, but there appears to be no power assist whatsoever. The vacuum assist unit is definitely hooked into manifold vacuum, as you can feel and hear the engine react when you press the pedal. In fact, after a long brake application, the engine (1987 Corvette 350 TPI) stumbles significantly, as if sacrificing great gobs of vacuum. The engine idles fine otherwise, although I have yet to check it at idle with a vacuum gauge
What you are describing is exactly what happens when the diaphragm in the booster has failed. If the diaphragm was good the engine would only react for a brief instant . I found a great booster made for hot rod apps on e-bay for less than $90 which works great on my T-bucket .

[kbuhagiar]

Originally Posted by adantessr

What you are describing is exactly what happens when the diaphragm in the booster has failed. If the diaphragm was good the engine would only react for a brief instant . I found a great booster made for hot rod apps on e-bay for less than $90 which works great on my T-bucket.

Thanks for all of the replies so far - I will be digging in to the problem first thing Saturday morning. Is there a test to determine if the booster/diaphragm is defective?

Adantessr, would you happen to have a link to more information on that booster for hot rod apps you mentioned?

Thanks again.

Cheers, Ken

[kbuhagiar]

OK, folks, did some inspecting and testing:

The booster doesn't seem to be holding any residual pressure. I performed a basic test (turn off engine, pump pedal to remove residual, re-start to see if the pedal drops). The pedal would not drop. Zero. I will start by replacing the check valve tomorrow. Looks like a standard item - hopefully.

Regarding the rest of the setup, I climbed underneath and got some information. The MC is stamped "29969", and I know from past experience that is a late 60's - early 70's GM application. The power booster measures 8" in diameter, and is stamped ABS. I believe ABS is a manufacturer of power brake conversion kits.

Not crazy at all about how close the exhaust pipe is to the power booster - the exhaust heat can't be any good for the booster.







Not sure if this is any sort of check valve or not, it is mounted inline in the front disc brake line:



The vacuum feed splits off at the transmission to feed both the vacuum modulator on the Turbo 350 and the power brake booster - not sure if that's kosher?



Thanks for everyone's advice up to this point. Appreciate any more comments and information.

Cheers, Ken

[adantessr]

Originally Posted by kbuhagiar

Thanks for all of the replies so far - I will be digging in to the problem first thing Saturday morning. Is there a test to determine if the booster/diaphragm is defective?

Adantessr, would you happen to have a link to more information on that booster for hot rod apps you mentioned?

Thanks again.

Cheers, Ken

I sent you a personal message on hotrodders on where to get the same booster as in your pictures . Did you read it ? I really don't think a check valve is going to do anything for you . You said the engine runs fine until you apply the brakes . You need a new booster . As close as the exhaust is to the booster , there is a pretty good chance the diaphragm is 'cooked' .

[kbuhagiar]

Hello folks,

This afternoon I confirmed the problem - a defective power brake booster. The replacement part is on order from a local supplier, I expect it by tomorrow. It appears that the reason it failed may have something to do with the close proximity of the exhaust pipe, it was actually in contact with the booster. That is unacceptable, so I have removed the entire exhaust system and will have it redone by a local rod shop as soon as the new booster is installed and the brakes are fixed.

Thanks to everyone who responded - I sincerely appreciate the assistance and support.

Cheers, Ken

[kbuhagiar]

Hello again folks,

OK, I replaced the vacuum booster - what a difference!
At least I don't have to stand on the brakes anymore, and my wife can drive the car now.

Follow-up question...I suspect that my MC lines may be reversed. The reservoir closest to the booster is connected to the front disc brakes, and the one farthest from the booster is connected to the rear drum brakes.

Could this make a difference? Is there a difference in the pressure output between the front and rear reservoir? I want to make sure I get the most performance possible out of the system.

Thanks in advance.

[adantessr]

Originally Posted by kbuhagiar

Hello again folks,

OK, I replaced the vacuum booster - what a difference!
At least I don't have to stand on the brakes anymore, and my wife can drive the car now.

Follow-up question...I suspect that my MC lines may be reversed. The reservoir closest to the booster is connected to the front disc brakes, and the one farthest from the booster is connected to the rear drum brakes.

Could this make a difference? Is there a difference in the pressure output between the front and rear reservoir? I want to make sure I get the most performance possible out of the system.

Thanks in advance.

With disc / drum brakes the larger reservoir goes to the disc brakes and the smaller one to drum brakes , regardless which is closer to the booster. It is very unlikely that there is any pressure difference , which is why many cars have a proportioning valve. My t-bucket does NOT have a proportioning valve with disc/ drum brakes and stops straight and quick . Only way I would worry about adding a proportioning valve is if the rear wheels want to lock and pass up the front in a panic stop. I'd take it up to about 40 and lock up the brakes in a panic stop and see what happens. It's better to know now . Glad I could help . Happu rodding .

[kbuhagiar]

Originally Posted by adantessr

With disc / drum brakes the larger reservoir goes to the disc brakes and the smaller one to drum brakes , regardless which is closer to the booster.

My MC is a Corvette style, with both reservoirs of equal dimension. But on the MPB website...
http://www.mpbrakes.com/uploads/docu...lumbingpdf.pdf
..they specify exactly which circuit goes to which port as if it would make a difference...

[adantessr]

Originally Posted by kbuhagiar

My MC is a Corvette style, with both reservoirs of equal dimension. But on the MPB website...
http://www.mpbrakes.com/uploads/docu...lumbingpdf.pdf
..they specify exactly which circuit goes to which port as if it would make a difference...

But doesn't a Corvette have four wheel discs ? The reason for the bigger reservoir on the disc/drum setup is beause the fluid level will go down on the disc brake side as the pistons go farther out as the linings wear . As long as you plan to check your fluid level regularly (every oil change ) and keep it topped off you should be okay. Just remember to draw some fluid out of the reservoir before you push your caliper pistons back in for for a brake pad replacement .