When I got a new caliper for my '69 350 I had to hold back the pistons with cable ties before the pads would go in. When I fitted new seals to a quite new caliper the seals were very difficult to insert. I eventually got them in but the pistons are stuck down as the seals are so tight, so that when I put in the pads I wouldn't need to hold them back. My worry is that when I fit the caliper to the car that either the brakes won't work because the fluid can't push the pistons or the brakes lock because the pistons won't return. Is it normal for the seals to be so tight, and will they eventually loosen up? I don't want to fit and bleed the brakes only to find they lock when I take it for a drive.

 

I use O-ring pistons in my coupes. When I did use lip seals, I used a kit to install the pistons. The kit consists of a stainless steel sleeve, one for the front and one for the back. I used the sleeve like a ring compressor on an engine.. Lubricate the sleeve and the bore of the caliper with clean brake fluid. Once the spring is installed, push the piston into the sleeve tool and locate the tool over the caliper bore. Push the piston into the bore. Install the seal on the piston and seat the seal into the caliper. I hope your caliper bores have been sleeved in stainless steel. That should do it. The fluid pressure will seat the piston against the disc pad after bleeding the air out of the system. Jerry

 

If it’s sealed and not cocked...that fluid will close that piston no problem.. the worry you will have including as stated above is leaks..assume you used lip seals..i have up and learned to go to orings and or rebuilt if not sleeved and smooth already

 

Are you using lip seals or o-rings? As TampaJerry asked, do you have stainless sleeved calipers?

Lip seals are, generally speaking, “looser” than o-rings. The lip design was engineered to allow the pad to slightly retract after the brake pressure was removed, the intended result being less wear on the rotor and pad. When brakes are applied, the fluid pressure forces the ‘lip’ radially outward, increasing the seal’s ability to do its job - seal against leakage. When cars sit for extended periods, fluid can leak out because the “lip” isn’t being pressed hard against the cylinder wall.

The o-ring design is such that a constant mechanical pressure is exerted on the piston and the cylinder and is much less affected by the increase in fluid pressure. The result is a “tighter” fit but lacks the same degree of ability to retract the pad. The upside is that the seal tends to be more effective against leakage.

That said, there remains a great deal of debate as to which is the better route to take. I have owned several C3s over the past 50 years and never had had a leaky brake caliper and I always have used lip seals.

One thing to keep in mind is that there are a great many choices between seal manufacturers and caliper rebuilders. It would not surprise me to have a situation like yours exist due to the cylinder bores being machined to the low end of the range (smallest diameter) and the seals manufactured to upper end of the range. Seal material characteristics will also vary - e.g. it’s Shore rating. Material from you know where is always high on the “be sure you know what you’re buying” scale. RMS of the machined surface also plays into the equation.

Shoot a pic of you caliper halves and post.

 

Did you merely install o-rings on your old lip-seal pistons? If so, that could be the issue with the very tight fit. The conversion kits I've seen in the catalogs all include new pistons, however, I've not seen them in person to compare dimensions with the lip-seal design piston.

 

I've never had O rings so I went for lip seals again. This time last year I noticed two calipers were leaking so rather than try to fit seals which haven't always been successful in the past I ordered new Delco Moraine calipers from Corvette Central. Because of the pandemic I couldn't get them anywhere and waited an incredible six months before they arrived in the UK. I fitted both but then noticed that the other two were leaking. The front one was reasonably new so I thought I'd try seals this time and got them from a Corvette specialist here. This is the one with the extra tight fit. Another reason for the seals is that the front caliper won't arrive at Corvette Central till April. The bores are just cast iron I think. The interesting thing that 69L88 mentioned is that maybe lack of use waiting for the calipers to arrive caused the other two to leak, because I haven't been able to take the car out all year. The pistons are fitted now so I wouldn't like to take them apart again, but I'm just hoping the tightness won't be a problem. The rear caliper is in stock at CC, but I'd rather fit seals to save time and money. Thanks for your help.

 

Ah. My apologies. I thought you installed o-rings in your existing calipers.