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Brake caliper seal design

Brake caliper seal design

Brake caliper seal design

(OP)
My question concerns the now rather old Girling type 16 series of brake caliper used on many UK-built cars in the 60s and 70s.

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.....

Repro parts.....!

Thanks

Nick

RE: Brake caliper seal design

Did you know that you can upload images directly inline in your posts with the "Upload Image" button at the top of the posting window?

RE: Brake caliper seal design

not an expert but have looked at this in the past, here is what I understand as well as a few questions

Based on the drawing my speculation is that the rotor is to the right of the seals as drawn. If that is indeed true, then my guess is that the extra chamfer edge at the open side of the seal grooves right (rotor) side is allowing the seal to deform more than required as it changes shape and is pushed against this wall. Which then results in the seal (and piston) retracting away from the rotor, but further than required.

RE: Brake caliper seal design

You could try anti knock-back valves Link
Or springs Link

You could also try machining out the chamfer leaving a wider groove. If you can't find a wider seal, it may even work with the original seal - tending to stay at the pad end of the groove.

je suis charlie

RE: Brake caliper seal design

"over-returns now being in the order of 3 - 4 mm per piston"

wow. The next stop would sure ruin my day.
What seals and pistons are you using ?

RE: Brake caliper seal design

(OP)
Mint Julep - that little trick had passed me by - thanks for pointing it out and re-posting the pic.

djhurayt - rotors/disc are to the left - I should have marked it on the sketch.

gruntguru - anti-knockback valves are an option, but shouldn't be needed. I've not seen the spring approach before. I have no way of machining the grooves. I did have a look through what turned out to be quite an extensive collection of alternate seal kits collected over a period of years (we have 3 cars that use these calipers in the family) plus some seals removed from OE calipers I refurbed a few weeks ago. There was a surprisingly large amount of dimensional variation, once even within the same kit - that one went in the bin! The repro calipers groove is already about 0.5mm wider than OE, but the OE seals and those removed from the repro calipers are the same width.

T-Moose - just finding my salvage plan had failed spoiled my day. Couldn't find any way to get anything that felt remotely like a brake pedal with those seals fitted, so didn't try a road test!
Pistons fitted are the ones that came with the repro calipers. These measure the same (with micrometer) as the OE Girling ones and have the same chromed ground finish. I do have a pair of usable OE ones I could try but doubt they'll make a difference.
I've now tried three seal sets
1- the ones that came with the calipers. They kind of work if you don't mind a brake pedal that bites 2/3 way to the floor (I do mind)
2- new Lucas TRW ones which are as close as you can get to OE these days. These measured the same as the above (as far as one can tell measuring rubber) but could be a little harder (very subjective as no way to properly measure)
3- used Girling ones of unknown age. These were visually perfect other having taken a slight compression set and measured slightly less depth as a result, but only ~0.2mm or so. I took care to fit these the same way round as their original install and they are delivering the best result so far, if a little short of what I know should be possible. Not real happy about re-using them either as the car they came off is from '72....... Still think finding some OE calipers to rebuild with OE parts will be the only sure solution.

Will contact the repro supplier tomorrow and see what they have to say. Also another company who may be their supplier.
The classic car movement is somewhat used to substandard repro parts but not so much in the safety critical stuff.

Thanks all for your thoughts

Nick

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