Power Brakes Ain't So Powerful

[kbuhagiar]

Hello folks,

Just purchased a 1947 Plymouth street rod, and I am in the process of assessing its needs.

The car has a Nova front end with disk brakes, and a Nova rear with drums. Very clean installations, no hack jobs. 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.

Thanks in advance for your thoughts and ideas.

[martybose]

You might just need to add a vacuum reservoir so that there is enough volume for your booster. There are a number of good sized aluminum or steel ones in the hot rod catalogs for all of the guys with lumpy camshafts.

Marty

Edited August 13, 2010 by martybose

[Frank Elder]

You might just need to add a vacuum reservoir so that there is enough volume for your booster. There are a number of good sized aluminum or steel ones in the hot rod catalogs for all of the guys with lumpy camshafts.

Marty

The beehive ones are pricey but sure would look good while doing the job!

[martybose]

The beehive ones are pricey but sure would look good while doing the job!

I actually bought a black steel one and put it sideways on the back side of a firewall brace below the passenger foot well; it's practically invisible there!

Marty

[randroid]

kbuhagiar,

A hole in the diaphragm could easily cause those symptoms. The vacuum draw from the engine is more than sufficient to run the brakes if the system is closed (no leaks) but continuous stumbling can only mean air is getting in.

Guess you're going to have to crawl under the car anyway, but changing a diaphragm is generally fairly straightforward.

-Randy

[Andydodge]

Also check the valve on the booster where the hose connects......I've blown the back off these valves and instant zero power assist tho' the brakes still work........lol........andyd

[chopt50wgn]

You may want to see if there are residual valves in the lines. There should be a 2lb for the front and a 10 lb for the rear. Residual valves should be plumbed into a disc/drum setup to help keep brake fluid from draining back in the lines.

[Don Coatney]

You might want to read this thread.

http://www430.pair.com/p15d24/mopar_forum/showthread.php?t=22469

Do you know where the master cylinder conversion kit came from? Do you know the bore size of your master cylinder? What is the size of your front calipers? Can you post pictures of the master cylinder instillation?

[james49ply]

Being a mechanic for longer than I like to remember, your discription of the engine stumbling (missfiring), is most likely caused by a vacuum leak, and since it only occurs when applying the brakes I would place the blame on the booster. Most likely a bad valve inside the booster itself. Have someone step on the brakes while you are near the brake rod entrance to the booster, if you hear a hiss, then definatly the problem.

[kbuhagiar]

You might want to read this thread.

http://www430.pair.com/p15d24/mopar_forum/showthread.php?t=22469

Do you know where the master cylinder conversion kit came from? Do you know the bore size of your master cylinder? What is the size of your front calipers? Can you post pictures of the master cylinder instillation?

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.

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.

Edited August 16, 2010 by kbuhagiar

[jd52cranbrook]

That check valve you mentioned looks to be a brake light switch.

[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.

[Greenbomb]

Not so fast- you're not done yet! After you get it all back together and drive it, let us know how it works. Thanks.