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Nick Jones

Repro calipers no good?

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I've moaned a bit about this before, but the story goes on.

When we upgraded the Spitfire brakes we didn't have any type 16 calipers to rebuild, so I bought some brand new ones.  Actual retailer probably irrelevant as I think they all come from the same place.

They looked good but the first problem was that we apparently couldn't bleed the system fully and just couldn't get a decent pedal - almost to the floor whatever we did.  Wasted loads of brake fluid.  Turned out not to be a bleeding problem but the caliper pistons over-returning.  Got around this by taking the pads out and pumping the pistons out, then pushing them back just enough to squeeze the pads and anti-rattle shims in.  Instant pedal.

Trouble is, the problem returns every 1000 miles or so (depends how fussy you are about pedal travel) as the calipers basically don't self adjust, so we have to remove the wheels, whip the anti-rattle shims out, pump the pistons out and refit the the shims.  We've stuck with it hoping they'd settle but they haven't.

I know we are not the only ones to have this issue - a local friend has an identical problem and have seem at least 2 more references on the forums.

Problem seems to be that there is too much stiction between seals and pistons.  There is mean to be a degree of "seal-roll" before they slip in order to pull the pads back from the discs, but there is just too much.

I had a set of "proper" seals in stock so today we pulled them off and swapped the seals.  Caliper quality looked pretty good and there wasn't alot obviously wrong apart from some mild scuffing on the piston/bore

P1180086s.jpg

and a thin black line on the piston, presumably where the seal had been sitting.

P1180088s.jpg

Seals, dust covers and pistons all in apparently good condition as would be hoped after a little over 1 year / 5 k miles.

Measure old v. new seals which showed slight variations, with the new ones being a little narrower (~0.1mm) but about the same thickness.  Old also felt a bit squashier.

Having fitted the new seals (on both calipers) with plenty of red rubber grease and reassembled everything the problem is........ worse :mad:

2 - 3mm roll-back now.  Unusuable.....:mad:   Suspect that the dimensions of the seal grooves in the caliper bodies are incorrect.  Will have to try and measure tomorrow......

Repro parts.......:down:

Nick

 

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As written in my last post in the Vitesse rebuild thread I seem to have the same problem.

BUT that is with reconditioned calipers and new seals !!!!

When I first rebuild the calipers I managed to tear a seal apart without noting.To solve the problem fast I ordered seals from Bastuck but these were thinner then to ones I had from the UK.

I seems that the thickness, width AND materiel need to be right for the groove in the caliper !!!

I think my best option is to hunt for NOS caliper seals. And Nick will probably hunt for original calipers and NOS seals …..

JC

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JC, the seal kit just fitted was a TRW branded one, so as close to OE as possible without being actual NOS (which could be really quite old now). Parts quality looked good and measured well compared to two old sets I had. 

Forced to dismantle again, we had a lot of trouble getting the pistons out. I could squeeze the pistons in easily a few mm by hand and they'd spring back to the start position when released. Likewise, a few psi from a foot pump would move them a few mm out but not further. We could apply up to 100 psi short term and have them return to the start point when pressure released. It was only when I left 100psi on there for 2 - 3 minutes that they slowly crept out.

we've done some comparing of the insides of these and an old OE calliper and also 5 different seal kits of various origins.

Conclusions are that the seal dimensions and quality vary considerably, but more significantly the seal groove in the calipers has a different internal profile and appears to be too shallow, resulting in too much compression of the seals. Our slightly ghetto (and hopefully temporary) fix is to remove-use the OE seals recovered from the GT6 callipers I rebuilt recently. These are in perfect condition but have compressed slightly to conform to the angled profile in the bottom of the groove.

By taking care to fit them in the same direction as they were we've ended up with pistons that move more or less as they should, though possibly still a little tight.

Brakes now working better than even, but remains to be seen how well they will self adjust.

Interestingly enough, the seals removed from the new callipers seem well made (definitely better than some of the kits) and similar dimensionally to the new OE ones, but softer compound, which is presumably how they worked at all.

Want some OE Girling 16PB callipers to rebuild now!

Nick

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Some pics

OE seal groove

P1180100cs.jpg

 

Repro seal groove

P1180092s.jpg

Dodgy not-to-scale-or proportion sketch showing the differences.  Regret that I have no way of measuring properly........ Disc to the left as drawn.

caliper seal groove.jpg

Pistons appear to be a good match in terms of major diameter and surface finish (ground then chromed).  All the groove IMO.......  Wonder if someone transfered a detail groove/seal design from a sliding calliper - which needs twice as much retraction?

Anyone had any that worked as expected?

Nick

 

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I also had one brake caliper on the workbench last weekend. Before taking the caliper off I checked how far I can push the brake pads against the brake disc.

On the caliper where I fitted a different seal I was able to push the brake pad about 6/10 of a mm with my fingers. All the other pads did not move when tested.

So I took this caliper off and replaced the seal from Bastuck for a TRW one. Brakes are better now but it still seems that the pedal get the brake pressure further up if you push it half way down once before applying the brakes. But its also possible that I'm all to fussy with the brakes now ….. 

1588290137_Build_up_20180818(10)forum.thumb.jpg.9353d0d2f519cc0d9601d4658d916da3.jpg

JC

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Should be possible to do better. My Vitesse now has very short brake pedal travel and that is with an "incorrect" 5/8" master. Callipers are cooked though!

I guess you already checked rear brake adjustment, front wheel bearing free play an disc run out? Maybe they'll settle in use.

Residual pressure valve is an option but should not be needed.

Nick

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The rear brakes are at the maximum, I even tried to adjust a wee bit firm but the brake drums were very hot after a test drive.

Front wheel bearings have just about notable play.

Did not check disc run out yet.

JC

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