Having problems w/C5,LT1 conversion on my Malibu

[white79bu]

I have been working on this darn thing since January. I have a 79 Malibu w/C5 front disc and LT1 rear disc. I have a brand new master cylinder from an 88 Monte Carlo SS. I installed a dual diaphram brake booster for better vacuum. The problem I am having is that the brakes are fine in traffic when you are using them alot and even in reverse. But when you drive the car a few blocks without using the brakes the pedal gets soft and goes down before the brakes kick in. I replaced the booster and master cylinder and that really helped. I have bleed the system so much that I have ruined my chrome lug nuts. If there is air in the system I don't know were it is. I am beat my head on the wall! I don't know what to do. I called Baer and the first time I talked to them they told me there was air in the system. So I bled the system and nothing. Today when I called they told me to cheak the vacuum. But I don't think that's the problem. I have never had a problem with vacuum. The only thing I have done to the motor was I went from a 750 cfm carb to an 800 cfm. Do you guys have any suggestions? Any help would be great. At this point I will do anything. Thanks Andy

[super74]

Isn't the master cylinder from the monte carlo for a front disc rear drum? The master cylinder might not have enough stroke for the brakes to work correctly.Check into a disc/disc master cylinder. I had a similar problem using a master cylinder from a 3/4 ton chevy but again that was a disc/drum master cylinder so it didnt help either. In the late 90's I was trying to set up 4 wheel disc with lincoln rear calipers and there wasn't alot of info out there, so needless to say I ended up scraping the project. I am going to give it another go with a set up similar to yours.Keep us informed when you get it sorted out.

[Steve68]

how about a vette master cylinder maybe early 80's

[white79bu]

super74: I thought the samething about the master cylinder. But Baer told me that they use the stock master in there kit for the Malibu. They even use a manual master cylinder for power brake set-ups.

steve68: I have thought about a master from a corvette too. I just may do that if I can't find something else. But Baer claims the master I am using is fine. But it may be worth a shot.

Thanks Andy

[super74]

Baer suggested the 3/4 ton chevy master with the 1 1/16 bore
same as my stock camaro one.But I've been told, that master will run out of stroke before the brakes are fully applied. I had trouble bleeding the brakes or so I thought. I was driving my car,the brakes worked semi decent,then while driving.....nothing! I had to drive through a red light,and back through my neighborhood with no brakes. I just let it idle home and up the driveway then slammed it in park.Please be careful until you get it sorted out. Master power brakes recommended the 70's vette disc/disc master to me to fix my problem, but my car is all taken apart right now. Let me know what you find out.

[white79bu]

Well I borrowed a vacuum gauge to see waht I am pulling. I am only pulling 14 pounds of vacuum at idle. So I called baer and they told me that I need more vacuum. What I can't figure out is why I did't have a problem before? But any way I am looking into an electric vacuum pump or switching to manual brakes. The car is a street car more than anything but I plan on going to some open track events and doing some autocrossing. So I don't know what would be the best way to go pump or manual. What do you guys think? Thanks Andy

[white79bu]

Well guys decided to bleed the brakes again before I bought a vacuum pump. There was still air in the system. I don't know were it could have been but it was there. It still didn't help. It's doing the samething. But I did notice that when setting in the garage and the car running that when you pump the brake pedal the rpm goes up. I would day around 500-800 rpm. That means there is a vacum shortage right? Atleast that is what I am thinking. I wanted to see what you guys thought before I go drop $300 on a pump. Thanks Andy

[essenge]

If I pump my brakes on my bone stock 74 camaro it raises the rpm's. I never thought that was a problem?

Did you ever answer the question about the master being for Disc/Disc not disc/drum?

[shortbed454]

the jump in rmp is normal. i dont think it a lack of vaccume. my truck runs 14lbs at idle. that is enough for the brakes. it sounds like the problem is in the master cylinder.

[white79bu]

I thought it was the master cylinder too. But I have talked to SSBC, Master Power, and Baer and they have all told me that 14 pounds of vacuum isn't enough. So I'm still up in the air on everything.

[Hydratech®]

Very simple - you are experiencing "caliper knockback". ??? Ok, that's nice, but just what in the world is caliper knockback? GM C-clip type rear axle = massive amounts of axleshaft inward / outward movement, which is knocking / pushing back the caliper pistons while simply rolling down the road. I'm willing to bet that you have no problems what so ever while the vehicle is sitting still, though suffer all kinds of inconsistency in braking once you get moving. What is happening is the axleshafts are moving in and out of the axle (especially when you make a turn), which is pushing the caliper pistons back into the calipers = the next time you hit the brake, you will have to take up all of that slack = low pedal !

What to do about this? The best answer is to yank the axleshaft C-clips and install (hardened) shims between the C-clip and the spider gears as needed to achieve about .010" axleshaft end play. The typical GM C-clip axle can in some cases be observed to have up to a solid 1/4" of axleshaft end play, which can knockback each caliper piston accordingly.

If you do not really wish to dig into the 3rd member (and do it the right / hard way), the alternate course of remedial action would be to install a residual pressure valve in the main line running to the rear brakes. Why? This will maintain just enough line pressure against the caliper pistons to prevent the pistons from being pushed back in, but not enough to create any real time drag on the brakes. The larger the amount of axleshaft end play = the larger amount of residual pressure you will need to counter the situation. Again, the right way is to correct the excessive axleshaft end play, though the residual pressure valves handle the situation quite nicely (and quickly). Contrary to conventional wisdoms, even a 10 lb residual valve will not produce any real time measurable drag / heat, yet will handily insure that the brakes will remain dead consistent each and every time you go to hit the brakes. Of course a 2 lb valve should be tried first before going into the higher pressure valves for obvious reasons. If you install a 2 lb valve and it is better, but still doesn't completely solve it, THEN swap out to the next "heavier" valve (like 5-6 lbs) - the 10 lb valves are only for the worst case scenarios (to where you REALLY should dig into the diff instead).

Here is a great article produced by StopTech Inc. discussing this issue: http://stoptech.com/tech_info/wp_knockback.shtml

Note that if you were running a C-clip eliminator kit or a 3rd member with press on type axleshaft bearings = this info does not apply (as axleshaft end play is very positively controlled = no in / out movement unless the bearing is smoked). Also, if the front wheel bearings are WAY out of adjustment, this condition can also occur in the front brakes, so double check your front wheel bearing adjustments too. The front wheel bearings being loose will usually also accompanied by strange behavior over uneven pavement, and will manifest as what seems like an odd alignment issue (and is most likely not a factor at all in your case).

Next on the list could be the MC you are running (!). The later model G body MC's are actually a bizarre design 3 piston design called QTU (quick take up). GM had such a problem with "mushy brakes" that they actually designed a high volume 3rd piston into the conventional 2 piston design MC's to help crispen up the brake response (which is why these particular MC designs have such an exaggerated large rear register size). The idea behind these designs is this: When you first hit the brakes, the high volume oversized rearmost piston is *supposed* to take up any slack in the system first, then a check valve kicks in to shut off the QTU function and then follow through with the forward two conventional smaller MC pistons to produce the pressures the brakes need. As you can imagine, this action is subject to various little metering / check valves inside of this type of MC working properly, which can malfunction with even the slightest little piece of dirt or wear. If the internal valves aren't working correctly, you can experience so many different problems that it would even be quite a challenge to list all the possible symptoms. My money is still on the axleshaft end play being your culprit though...

BTW - Hop into any EFI equipped late model ride (equipped with vac assist booster) and hit the brakes quickly and release in rapid succession as fast as you can. You may be suprised to see that the RPM will increase rather dramatically! Why? Every time you let off the brakes in a vac boosted brake system, you are actually flowing air into the intake manifold = idle speed will increase. Sounds funky? Not really, as it just how the vac boosters function...


:cheers: