A bit of a hard one to explain,

I have princess 4 pot callipers on the front of the 7, with green stuff pads
new brake fluid
pads all copper slipped

If braking hard, the os brakes snatch and lock, but releasing the brakes slightly both seem to brake consistently and in a straight line

So, I whipped the pads out on the os calliper first, and with a wedge in place of the pads pumped the pedal, both lower pistons come out first, and once at the wedge, the upper pistons come out,

Just wondered if this seems correct, as I would of thought that the would all come out together

I havnt had a chance to do the ns one to check if it does the same, garage is too small! but that is he next thing to try

Any one had a similar problem

steve

quote:

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Originally posted by steve m
A bit of a hard one to explain,

I have princess 4 pot callipers on the front of the 7, with green stuff pads
new brake fluid
pads all copper slipped

If braking hard, the os brakes snatch and lock, but releasing the brakes slightly both seem to brake consistently and in a straight line

So, I whipped the pads out on the os calliper first, and with a wedge in place of the pads pumped the pedal, both lower pistons come out first, and once at the wedge, the upper pistons come out,

Just wondered if this seems correct, as I would of thought that the would all come out together

I havnt had a chance to do the ns one to check if it does the same, garage is too small! but that is he next thing to try

Any one had a similar problem

steve

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My thoughts too.

When setting bicycle rim brakes up you set them up with a slight toe in to prevent them squealing and improve brake performance. I usually use a thin bit of card between the rim and the rear edge of the brake pad to get the toe in (if that makes sense).






[Edited on 2/10/18 by nick205]

Are the 2 "halves" of the callipers linked (most seem to be) as originally each half (top/bottom) would have seperate feeds off a dual master cylinder.
But assuming yours are linked, they should all work at the same time.
Not being rude here, but lack of use causes havoc with brakes. You may just need to pump the pistons in and out a few/many times to ease them all off.
It sounds like the NS pistons may be slow to move? And perhaps a bit sticky?

Not the case here, but some reproduction new callipers seem to be slightly incorrectly made (what a surprise) with the calliper grove machined ever so slightly wrong, meaning the pistons get gripped too hard. And not at all obvious what the issue is. But as I said I digress (it is my lunch break!)

I think the piston with the least resistance would just move first till it ran out of travel or found more resistance and then the next one would move and so on, it's very unlikely all the piston frictions would be so perfectly matched for them to move at the same time.

Once the pads are in the movement is very small so they will move almost at the same time. It's more likely you have contamination on ether a pad or disk or that one or more of the pistons is jamming.

Have you ever had the car properly corner-weighted? A significant imbalance will cause premature locking on one corner.

Some interesting replies, thanks guys!

But Mike RJ has hit a chord, as no I havnt, and it was always one of the things I thought were a possible reason for the snatch/lock

But also, I will check to see what the ns is doing, and similar, and also check the leading edge on all the pads are chamfered to give equal lead in time, this is also a possibility !

regards

steve

I'd go with Mr Whippy and check for a seizing piston

Definitely a seized piston or at least 1 with more friction than the other.
Without any restrictions in the the pistons brake fluid will exert the same pressure on all the pistons at the same time as it is a sealed system.

So, today I did a similar test to the ns calliper, and at least one lazy stuck piston, so calliper taken off and stripped down to examine

Two pistons corroded, not bad but not good! two possibly reusable,
There was lots of crud in the galleries etc and bits, of what I don't know, but greasy slimey sludge and bits

o/s also taken off, and disassembled, and appeared to be in a much better condition, but again a couple of pistons were below what I would accept, the other two perfect

Ive ordered a whole set of pistons x8 plus seals, and will rebuild both callipers, and will repost the results

steve