Solving a Rear Brake Dilemma
I WAS READING the November Mechanic on Duty entry “A Difficult Rear Brake Dilemma” where Mike Huelsman said the factory rear disc brakes on his 1989 Pontiac Firebird Formula “don’t seem to hold well at all.” I know exactly what the issue is with his rear brakes.
Mike has one of three Delco Moraine single-piston rear disc brake calipers, all of which work on the same principle.
GM introduced these calipers in the late ’70s and used them into the late ’80s. The three different designs were basically two different-size models of the old Delco exposed sliding pin design and an aluminum version used on the Fiero.
Mike has the smaller exposed sliding pin caliper on his Firebird.
Mike’s problem is a very common one. The caliper is not adjusting properly; much like a rear drum brake with the shoes not adjusted. And I’ll bet his parking brake doesn’t work well either.
These calipers have a compound piston assembly called an actuator. (I have included a picture of one disassembled.) There is an inner piston and an actuator jack that are supposed to work independently of the piston shell. But most of the time, these do not work as they are supposed to. When they don’t, you lose the self-adjusting affect this is supposed to create. Even rebuilt calipers seldom work properly. Only replacing the actuator assembly with a new unit will get these working properly and new actuators can be hard to find.
So, the best way to deal with this is to simply adjust them manually.
The primary adjustment can be done in one of two ways. You can screw the piston out by turning it counter-clockwise until the caliper, with pads in place, just slides over the rotor with no drag. Or you can remove the parking brake lever nut from the rear of the caliper; turn the lever manually in the lockup direction, pushing the piston out as far as it will go. Then remove the lever and reposition it in the farthest unreleased position you can. (Don’t turn the jackscrew back while you reposition the lever.)
Once this is done there is one more thing you can do to help with the adjustment. Looking through the hole in the top, you will see a groove in the edge of the piston. Have someone hold the brake pedal down tight, insert a screwdriver into this groove and push out on the piston, attempting to hold it in the “brake on” position. Then have them release the pedal. Do this several times to get the inner piston to adjust and take up any remaining slack.
This is a real pain but most times once the adjustment has been done, the caliper can function for a long time without needing readjustment.
Remember that the “D”-shaped pin on the back of the inner pad must fit into the “D”-shaped hole in the piston. (See the picture.)
Also, bleeding these can be difficult. Tapping the caliper and working the parking brake can help release air trapped in the actuator assembly.
One last thing, don’t expect the rear brakes to work like the front. GM proportioned the rear braking power down on these cars to prevent rear brake lockup. The best way to tell if the proportioning valve is working like it should is with a set of brake pressure gauges and the specification from a shop manual. I would expect something like an 80% front to 20% rear range.
Mike Evangelo Cincinnati, Ohio