Hey all, I'm beginning my research and parts acquisition to ultimately swap from drum to disc on the front of my '68 4-4-2. Through my extensive reading here and elsewhere, I have learned that the '67 and '68 calipers were 4 piston units, while the '69 and up units have a single LARGE piston. "Correct" would be locating and using the 4-piston calipers (Rock Auto, etc.)..

'67-'68 4-Piston Version'69 & Up 1-Piston VersionI already have the single piston units - I got them from a member here - THANKS! I will be tearing all the assemblies down, cleaning, blasting, de-rusting, then applying brake application paint; all before installation.

Does anyone have the low-down on whether there is/are any functional or mechanical efficiency (safety?) differences between the 4-piston VS the single-piston setups? Enough to lead me toward installing the later 1-piston unit instead of the year-correct 4-piston unit?

 

There are several reasons why GM went from the four piston to single piston calipers. First, the design of the four piston caliper has rubber seals in grooves on each piston so that the seal slides on the bare cast iron bore of the caliper housing. Any water in the brake fluid (and brake fluid IS hygroscopic) causes rust on the bore, which chews up the seal. These calipers were notorious for leaking. The single piston caliper uses a seal fixed in a groove in the bore and the chrome plated piston slides in the seal. The plating make rust a much less common problem, thus fewer problems with leaks. Yeah, you can get the four piston calipers sleeved with stainless, and that helps a lot, but it's an extra cost.

Second, since the four piston calipers are fixed, any runout of the rotor causes "knock-back". This is where the pistons get pushed back into the bore, increasing the gap from the brake shoe to the caliper friction surface and thus increasing initial pedal travel when you step on the brakes. The single piston calipers float on pins, so rather than forcing the pistons back into the housing, any rotor runout simply causes the caliper to slide back and forth on the pins.

Third, the single piston calipers are much cheaper and since they are one piece, there are far fewer potential leak points.

Bottom line is that unless you really care about correctness, I'd keep the single piston units. FYI, the four piston calipers require different caliper mounting brackets, different hoses, and different pads.

 

I think it depends on how correct you want the car to look. I agree with Joe that the single piston calipers are cheaper, more common and potentially have less issues. But when I'd picked up a set of the four piston ones for a project car I talked with yellowstatue about this. I'm not finding the post, but he gave me a reference of who could install the stainless inserts and shared he's not had problems once the inserts were done. These were also used on other GM cars although I believe there are casting numbers/piston sizes that differed between some of the applications. John

 

Hmm... I thought I had replied to this thread yesterday.

Thanks for the input and information guys, I appreciate it! I'll do a search and see if I can find the post from yellowstatue about the inserts, but will likely keep and use the single piston version for now.