Brake caliper seal design
Brake caliper seal design
It's a robust and reliable (if rather heavy) cast iron design with two opposing pistons of 54mm diameter.
The pressure seals are simple, rectangular section rubber rings, one per per cylinder and there is an external rubber dust/weather seal. The dust seal design has varied over the years, but the main seal design hasn't.
Though the seal itself is very simple, the shape of the groove it sits in looks to have some subtle detail, which is no doubt there for good reason.
In recent years reproductions of these calipers have come on to the market at reasonable prices and on the face of it they seem to be good copies. However, I and others have had problems with them.
Specifically, the pistons over-return when the brake pedal is released making it impossible to get a good pedal feel on initial bleeding or sometimes any pedal at all. It is possible to get around this by removing the pads, pumping the pistons out a little, then easing the pistons back just enough to get pads and anti-rattle shims back in. This gives a good pedal feel, but the problem is that as the pads wear even over relatively few miles, the caliper does not self adjust until the pedal is again far too low. They can of course be "manually" reset, but having done this a couple of times in the hope that things would settle I'm not prepared to do so again as there is clearly a fault. As project build time and lowish mileages covered mean we are well outside the official warranty period, I decided to dismantle the calipers and fit a genuine Girling (now Lucas TRW) seal kit. The calipers were in apparently perfect condition inside with assembly grease still present. Unfortunately the new seals made matters worse with over-returns now being in the order of 3 - 4 mm per piston and 100 psi pressure being needed to re-extract the pistons (greased with silicone rubber grease supplied with the kit a few hours before hand).
Measuring the the original seals and the Girling ones didn't give many clues as dimensions were very close. The original seals were possibly slightly softer, though measurement methods were rather subjective.
While apart I compared the calipers with an original Girling one and found them to be faithful copies in all respects except the shape of the groove that the main seal sits in. The attached sketch shows the differences.
My understanding is that the shape of the groove is intended to give some pressure activation of the seal, and also to provide a certain amount of seal "roll" before piston/seal slippage occurs and it is this roll that intentionally retracts the piston a small amount to prevent brake binding. It seems that in this case we have too much roll occurring and thus too much retraction.
I'm not sure whether there is a simple dimensional issue, with the seal groove being too shallow, leading to over compression of the seals, or whether the altered shape of the groove has an influence? I wonder whether the manufacturer, most likely reverse engineering from samples rather than working from OE drawings, has opted to use a design for which they do have drawings, but this design is from a single piston, sliding caliper which will need something like twice the retraction.
I will take this up with the suppliers and would welcome any insights from someone who actually knows about these things rather than just speculating as I am.
Suspect I'll end up dumping these in the recycling and finding an original Girling pair to rebuild though.....