Hey, any ideas as to why when I converted the front power drum brakes to power discs on my 70 Chevelle, that I have a very firm pedal, but lousy braking (demanding extreme pedal pressure)? I have replaced the booster twice. The pedal does not sink, I have pressure bled and gravity bled the system, and a local mechanic has bled the system, and saw no air. Could the culprit be the distribution block/proportioning valve? Is there a different block/valve for disc/drum set up as opposed to drum/drum set up? Thanks for any input!

That is a bit strange. I thought you may be using the drum type master but after looking up the specs I see it has a smaller bore wich would give you the opposite effect of a soft pedal but good brakes. The only two thing that come to mind would rod adjusment and pad material. Check to make sure the rod that exits the booster is not to short. This could cause the stroke to be to long and "pass thru" the power section of the booster effectively giving you no power brakes. Some of the newer pad materials take some serious breaking in before they seat. For some reason I have a feeling none of these are going to be your problem! I have a feeling you are on the right track when you supected the proportioning valve. You might consider bypassing it all together and installing an adjustable unit. Just bypassing it should tell you if it is the problem or not. Good Luck

The proportioning valves are definitely different! Very well could be the problem!

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Tom Parsons

As Tom said, your "proportioning valve" could be the problem, because you don't actually have a proportioning valve in your system. With 4 wheel drums, you don't need a proportioning valve, only a distribution block. When you convert over to front power disc brakes you need the proportioning valve to equalize the application of your braking system, front disc to drum rear. Pick up a combination valve from Ground Up (our sponsor) and then try it again.

I have a similar problem with my car..
I am running '72 Monte Discs on the front and '79 Trans Am discs on the rear. I took the proportioning valve off of the Trans Am but I still have the same symptoms as '70Chevy mentions-very high pedal effort and minimal braking action. The car stops from high speeds well but when you are trying to stop from like 5 mph it requires mucho pressure.
I am thinking that my master cylinder is wrong for my application. But, which one do I purchase? I don't want a cast iron piece of junk off of a '79 Trans Am. Would prefer a later model aluminum plastic one off a 3rd gen F body w/4whl dsc.. Anybody know if this will work?

For 4 wheel disk brakes you need a larger bore master, 1 inch or larger with a fair size reservoir, dual. They require more fluid and higher pressure. There are a couple ofthe F body units that work. I think Mark Williams has them and I know Lamb Components has them.


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Wally
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[This message has been edited by Wally (edited 06-14-2000).]

KEWL! Guess I could get one from the junkyard and try it out and see if if improves my current dilemma.. Man what a dirty job bleeding all four corners. I guess the other alternative of buying a complete new front end after I rear end someone is not so appealing either.. Choices, Choices. I will keep you guys posted as to the results. I am definitely going the 3rd gen Fbody M/C route. The cool thing about them is that the fluid reservoir is angled so that it sits level once mounted to the power booster. Much better if you ask a detail fanatic like me.

Sounds to me like your booster is not working.
Try getting in the car pump the pedal several times then hold the pedal and start the engine,if the booster is OK the pedal will sink,there is a small vaccuum check valve that goes in the line and in the booster,be sure it is OK...FRED

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WOW, I have got the same EXACT problem I'm working on now. I replaced the front brakes with Stainless Steel PBR calipers with slotted 12" rotors, new booster and big master cylinder. Then I got a new combo valve and lines from Master Power. I put in all new Dot 5 fluid and bled 4 times. The booster is good, the master seems okay. About the only thing left is the new combo valve. I got a new pressure gauge kit to check each corner for pressure, maybe that will point to the problem. Let me know what you find out with your problem. Brakes like this are very dangerous!

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1970 Chevelle "SS502"
502-540HP-560ft-lb
1992 Taurus SHO "please buy so I can get another Chevelle"
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Team Chevelle Member #521, ACES #1994

A buddy of mine had the same exact problem when he did the drum-to-disc conversion. The culprit? The proportioning valve. Troubles went away after he installed the correct valve.

A week later, he hits the brakes hard to avoid a collision in the intersection in front of him. The car in front of him got smacked back. And he, unfortunately, got rear ended and his car was totaled out. The irony, huh?

Robert