Front brakes not releasing

[Forkie]

As in the braided Goodridge hose?
So further to my earlier post today - got her started up this eve, to nudge up the drive, then reverse back with the O/S closer to wall, to give better access to the N/S, just for testing movement of the N/S/F. ( did contemplate earlier today re putting her back on the skids for ease of moving around) .
So - once better access gained to said N/S/F - all i did this eve was purely jack that wheel up to get a feel for movement - exactly the same as the O/S /F - tightish, wont spin.
So tommorrow's plan is same checks as for the O/S, then up on to the ramps for a visual of the underside, concentrating on the gearbox area
I did assume earlier that as both front corners had exactly the same treatment, at the same time, if one side had developed a fault and the other not, i would of felt a deffinate brake imbalance. Which i havent.

[Forkie]

So i have had fore and aft up on ramps - nothing wrong noted - no leaks from anywhere, no damage to any pipework - albeit tricky to see up in the tunnel but had a good look. So i assume i have something not quite right in the pipeline somewhere, so will presume have to start at the flexis, and go from there.

[Clifford Pope]

You have I take it set up the pistons with what the manual calls the "relieved" section of the outer face correctly aligned?

I have never understood exactly what these sections are for - is this something to do with encouraging the pistons to retract?

Come to that, how can a seal exert a return force ? I presume if it were installed back to front then it would cause the piston to creep outwards instead of retracting?

[Forkie]

Hi Clifford
Im not sure either, how a seal on the piston causes the retraction - but what else does? The calipers i purchased as rebuilt units ( frow CW i am sure , would have to check the mountain of bills in the pile) around 15 years ago on an exchange basis of course, so i never actually fitted the pistons myself. They do look aligned as per the manual, but i may well double check that.
Incidently it appeared obvious on removal of both wheels that the inboard side of both calipers were showing copious amounts of pad dust, hardly anything outboard - so assume the issue is with the inboard side, on both sides. Not quite sure how that situation arises - i would of thought both sides of each caliper should exert the same force.

[johnnydog]

I'll let you decide on the reasoning behind this Nige as to the function of the piston seals to slightly retract the pistons after a brake application to prevent them binding.....copied from t'internet by the way. Refers to sliding or floating calipers (with a single piston), but the jyst of it is the same principle as our two piston calipers.

"The square cross section caliper seal does indeed retract the brake caliper pistons. I used to work for GM’s Delco and the brake engineers told me that is how they retract the pads to reduce friction and increase fuel economy. The slight wobble in the brake disc due to warp and bearing slop centers the pads on sliding caliper designs for the least friction. The downside to all this extra clearance is that the brake pedal has to move further to get the initial brake apply. They did all kinds of silly things like stepped-bore master cylinders and such to take up the initial slop. You can also use a bigger master cylinder bore (which applies less pressure) with a greater boost ratio (for increased force) from the servo to compensate".

And from the same site...

"The caliper piston seal is of a “square cut” design. The outside edge of the groove (the edge of the groove nearest the inner brake pad) is cut at a slight angle. This angle allows the seal to flex slightly on brake application.

Then when the brake is released the seal then returns to its normal shape pulling the piston back to its original position. If the pads have worn enough, the seal will actually make contact with the angled edge and stop. The piston will then continue to move, slipping on the seal allowing the piston to adjust for wear.

The seal does retract the piston".

So whether this goes any way in answering the question?

[Forkie]

Umm yes, in a way i suppose, that explination regarding how the seal retracts the piston back from the disc makes sense. What doesnt make sense, in my case alone it would seem, is why that would only occur on one half of my calipers, x 2 , and not on both sides....x2 ......

[Forkie]

I mean , i cannot be the only one to experience this issue, surely? After 50 years? I just assumed there was a common problem, that i am not aware of, as to why the pads are seemingly stuck on ( on one side of each wheel) through non retraction, of the inboard pad? In short - i should be able to jack up a front wheel, either side, and be able to spin the said wheel - without any hinderence, and it should be free to spin?

[Alec]

Hello Forkie,

another posibility I have just thought about, are the pads\calipers corroded where they sit, i.e. they are tight in the calipers due to rust?

As for pad retraction, Jaguar were an early leader in disc brakes for their road cars, they were made by Dunlop and had a shouldered pin in the centre of the piston, and a slotted steel plate attached to the back of the pad. This pin engaged in the pad and was though, at the time, necessary to retract the pad after application. This proved to be uneccessary.

Alec

[Forkie]

Hi Alec
No rust issue at all, as everything is brand new and spotlessly clean. Had a chat last night with our tech guy that looks after 'the moderns'. With what ive described, he possibly thinks there might be an issue with the feed through the calipers to each half. I believe the plan is to see if i can get the car over to his workshop for him to have a better look at things over the weekend. As nothing is glaringly wrong, he cannot make much judgement without actually seeing for himself.

[Alec]

Hello Forkie,

I had to go back to post 1, where you had alrady removed the pads, which discounts my last suggestion.
Standing for 15 years with hardly any use makes me believe that it is internal corrosion in the cylinders. Lack of feed from one side to the other would be very rare and also should that be so very dangerous as uneven pressure on the disc can break it if sufficient force is used.
Hopefully all will be resolved very shortly?

Alec