Supercharged AJP8. one last step!

[gruffalo]

FarmyardPants said:

They look great. I need a brake upgrade, my AP 4 pots have never been the same since being refurbed, weirdly enough. The above wouldn’t fit inside my 17” wheels though.

After the refurb I assume you put new pads is, if so what?

Pagid RS14 suit the Cerb brilliantly giving incredible feel and control whilst lasting very well and giving huge retardation.

[FarmyardPants]

Yes, new pads (green stuff). It’s the calipers, the piston seals pull the pistons back in after each use, giving a long pedal. Worse still, the retardation is barely above MOT limit. In an effort to fix it, I fitted springs behind the pistons and used lots of red grease during reassembly but they are still marginal. I could only get the 4lb springs (the weakest of the 3 that AP sell), and only one of the 8 pistons comes out by itself, if you see what I mean. Thinking of ditching the whole setup.

Edited by FarmyardPants on Sunday 6th August 09:32

[a1rak]

Only trying to be helpful here

So if you take the dust seals off does it solve the issue. Nearly every road car caliper has dust seals and don't have issues.

Are you 100 % sure you don't have a warped disc somewhere on the car. I had this and it caused the exact issues you have. Id clock the discs for run out and thickness when then are secured firmly on the hub. When you bleed and test the brakes on a stationary car they seem perfect. As soon as you go down the road you have a soft peddle.

If thats all perferct it might be worth replacing the master cylinder.

Got to be absolutely sure its totally bled well. 1 small bubble and you're stuffed.

Did you use a pressure bleeder because they save a massive amount of hassle.

Anyway good luck in solving the problem. Keep us posted.

[FarmyardPants]

Thanks. Master cylinder was changed. Discs are not warped, although I could do with fitting some new ones (330mm Brummmie specials a few years back). Wheel bearings are ok too. The effect is there even without the car moving, clamping the front brake lines gives a good pedal so it’s definitely the fronts. The pistons don’t have dust seals. Might have to try to get hold of the tougher springs and fit those, which is a ball ache.

[ridds]

Did you get the right size Master Cylinder?

A smaller cylinder would give high pedal travel and allow easier return of the fluid, due to the effective area of the pistons over the Master Cylinder.

Although this would be strange without disc rotation.

I can't see how the seals return the pistons as my seals were just plain square section.

This the seal kit you fitted?

[FarmyardPants]

Yep. New seals and pistons for all 4 calipers. Rears are ok. And the correct master cylinder, which was changed to eliminate it as a potential cause but had no effect.

Edited by FarmyardPants on Monday 7th August 18:08

[ridds]

Maybe it's all in the springs then. 4lb is quite different to the max of 12lb.

Wonder what was originally fitted to them.

Still can't get my head around how they return on their own though without the disc helping them.

[SlimJim16v]

I believe you can get a valve that goes in the brake hose/pipe that keeps some pressure in the calipers.

[FarmyardPants]

SlimJim16v said:

I believe you can get a valve that goes in the brake hose/pipe that keeps some pressure in the calipers.

That’s an idea. I sent them away for refurb and apparently they didn’t have any springs previously. Valve seems easier than taking the pistons out again

[Basil Brush]

The original spring rate is the last digit in the caliper reference so I think 4lb is correct.

[julian64]

I think I'd be looking closely at the master cylinder. I've never seen a dust cover powerful enough to pull a piston back once you'd taken your foot off the brake pedal. The master cylinder plunger should deform on negative pressure to stop any vacuum, and it sounds like its keeping a tight seal on the backward stroke. Not what it should do.

Possibly check that the brake reservoir cap isn't forming a pressure seal and is letting air in.

Edited by julian64 on Wednesday 9th August 14:08

[ridds]

julian64 said:

I think I'd be looking closely at the master cylinder. I've never seen a dust cover powerful enough to pull a piston back once you'd taken your foot off the brake pedal. The master cylinder plunger should deform on negative pressure to stop any vacuum, and it sounds like its keeping a tight seal on the backward stroke. Not what it should do.

Possibly check that the brake reservoir cap isn't forming a pressure seal and is letting air in.

Edited by julian64 on Wednesday 9th August 14:08

That's a good point. A fully sealed cap could cause this. Tried the brakes with the cap off? (Probably)

[FarmyardPants]

Good point, will try with loose cap. Thanks.