As the heading states, why did Pontiac use phenolic pistons in the pre-88 front brake calipers?

I am rebuilding all of my calipers with new seals & hardware. The front brakes are strange to me as the phenolic pistons do not slide easily in the caliper casting. Even without the new seal in place, they want to lock if they do not go in perfectly square and they are a "push-fit" at best and not a "slide-fit". I have an extra set of front calipers to play with and have mix & matched the pistons. They all fit together about the same.

Also, there is a rusty metal cap on some of them. What is the purpose of this cap?

Gordo.

They used those pistons because they do not transfer heat which is bad for brake fluid. It boils then puts air pockets in the fluid which means spongy pedal. If you can't get the piston in the bore may be messed up. Get some remanufactured calipers.

[This message has been edited by Crow (edited 10-01-2005).]

Got four front calipers & they're all the same. They go in, but not easy like the rears.

The metal cap prevents wear and chipping on the front of the piston. I don't know how critical it is.

The phenolic piston has different clearance requirements than the steel ones. They usually fit pretty close but if it's binding the caliper bore may need work. Leave that to a rebuilder that has the tools to not make problems worse. Honing an Aluminum caliper is really easy to screw up due to how soft the metal is.

GM and other car makers use the phenolic for both performance and manufacturing reasons. Besides the thermal insulation you gain... The phenolic comes out of the mold ready to use. Metal ones need machine work and polishing before they can be used. Contrary to popular myth the phenolic pistons aren't bad but you cannot treat them the same as metal ones. Doing that will cause problems. Example, 80's Chrysler vehicles had a reputation for problems with Phenolic pistons that is usually amd falsely blamed on the pistons. The real problem is the pistons weren't given enough clearance in the caliper bores when the factory built the calipers. Just a tiny amount of trash buildup in the caliper would make the pistons bind.

When installing either piston type, especially in new seals, make sure you lube the seal and piston with fresh brake fluid. Failing to do so can damage the seal.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurasic Park)

The Ogre's Fiero Cave (It's also at the top of every forum page...)

i just sent in some rears from an 84 to TFS for their core charge of $30 each and they both had metal pistons

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92 BMW 325i
86 Fiero V6
AIM:Onefast2M8

Fiero rear always has metal pistons. (read the rear brake article in my cave about how those are built.) Factory front ones are Phenolic.

I went through 4 pairs of calipers with the phenolic piston and did not get one that did not bind or stick. The car would dart to one side of the road when you hit the brakes and then to the other when you let off. I finally gave up and ordered calipers with the metal piston. That was about a year ago and so far I have not had any trouble with these calipers.

Thanks for all of the responses. I have rebuilt all of my calipers with new seals etc. After a short test drive, it appears that the right front caliper sticks a bit. The wheel feels warmer than all of the others and you can hear a bit of a creaking noise when you take off again. I think I will replace the entire caliper. It's actually cheaper to buy a remanufactured caliper than to buy a new piston.

Make sure it's not something less obvious....

Example... the inboard pad can bind between the arms of the bracket. The caliper itself can bind between the arms under certain conditions.

I've seen where you need to remove stamping burrs or paint from the ends of the inboard pad more than once. Even on premium brands.