March 2012 Chevy High Performance Q&A

What’s going on out there?

Kevin McClelland Jan 20, 2012 0 Comment(s)
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Malibu Brake Problems

Q. I replaced a 41-year-old set of drum brakes on the front of my ’70 Malibu with a disc brake kit from PST, along with new ball joints, tie rods, centerlink, springs (front and rear), control arm bushings, and cylinders and shoes on the rear drums. We removed the distribution block off the frame and put brass-flared fittings in its place. The kit came with a proportioning valve, master, and power booster.

The booster clevis is in the lower hole in the brake pedal, the sleeve is in the master cylinder with the short pushrod in the booster, and the rear shoes are adjusted up. The control arms went together with the new ball joints, rotors, and calipers. Upon bleeding the system, we could not get a pedal to come up and thought air was still in the system but could not get any air.

PST sent us a new master, thinking that was the problem. It did not help. The 350 engine has a slight cam; I don’t know the duration. The vacuum is running 15 at idle with the engine at normal temperature and 20 at 2,000. The booster feels like it is working; the pedal goes almost to the floor and then gets firm about an inch off the floor with good stopping power. It is not spongy but firm at that point.

I took it to our Chevy dealer to have the frontend alignment done. They checked it for air and said it didn’t have any in it. They told me the cam was not letting the engine produce enough vacuum to operate the power brakes and suggested I put a vacuum canister on to help boost the power brakes. At this point, I had not replaced the proportioning valve; I changed the booster with no change in the pedal. Do the front calipers travel enough to require that much pedal? They should not!

I have never in the 40 years I’ve been working on Chevelles or any other car run in to this kind of problem. It has literally got me stopped in my tracks. Please help!

John Moore
Trenton, SC

A. Something as simple as hydraulic brakes can sometimes be a real headache. It sounds like you’ve done everything right. There are a couple of hints in your letter that may give us a direction. You’re spot-on that the front calipers shouldn’t require that much pedal travel unless they are the quick-take-up G-body metric calipers that were used on ’81-and-up G-body GM cars, for reducing rolling resistance to increase fuel economy. These later calipers used a special step-bore master cylinder, which had piston sizes of 24mm/36mm. The larger piston would fill the calipers quickly and the small 24mm piston would give you the low pedal effort. The PST website says its kit is supplied with 72mm calipers. Most of the factory-style disc brake kits on the market today use the later G-body-style calipers because they are cheaper and easier to get. The later-model G-body calipers require more fluid to bring the pads in contact with the rotor. The factory G-body calipers are much smaller than the 72mm supplied by PST, and the early factory Chevelle single-piston calipers are 215/16 inch (74.6 mm). Check what size master cylinder you have. You should be running a 11/8-inch master, or the later G-body–style caliper.

The fact that you have a firm pedal an inch off the floor, and good braking at that point, tells us you have plenty of vacuum for the booster—15 inches of idle vacuum is plenty for power brakes. You said the “sleeve is in the master cylinder with the short pushrod in the booster.” You should be able to feel a slight bit of pedal movement before the booster pushrod begins to move the master. You don’t want more than about 1/16-inch clearance between the booster pushrod and the master cylinder piston. We think you’ll find your problem in the master cylinder sizing. Good luck, and be careful until you find your pedal.

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