Front brake overhaul question

[pamitchell]

Should the piston bores be lighly honed when installing new pistons?
I'm chasing a front brake squeal. I've serviced the front brakes in the last 2 yrs w. new pistons, seals, clips, pads, rotors, pins, applied anti squeal to the pads & retained the anti squeal shims.
The brakes are original w. 90,000 mi.
Since the overhauls, the brake feel and travel improved and the squeal has diminshed.
I read the brake squeal thread and will also try putting silicon sealant on the pin ends and checking the front bearing pre-load.
Is honing the piston bores recommended?
TIA

[YellowS4DHC]

Hi Phil

when you say "checking bearing preload" you mean "ensuring there's not excessive end float," right?

I believe the WSM requires 2 to 4 mils end float after which you back off a flat and insert the cotter pin. So technically, the front wheel bearings aren't preloaded.

not trying to pick nits or annoy, just want to be sure I haven't misunderstood the WSM's caveat about bearing end float.

(btw, we must be fairly close geographically, I'm in s. san jose near almaden exp and blossom hill rd)

regards
Rick


'69 Elan S4 DHC
'57 T1 Speedster
'67 S800 Coupe

[UAB807F]

I've never honed the caliper bores when I've replaced pistons, unless there's obvious damage I can't see any reason for doing it. (and if there was damage to the caliper bore then I'd just scrap it and buy new).

If you've got the shims in place, everything is newly re-built and applying even pressure then I can only think of changing pad material as the next stage in solving the problem. The last set of pads I bought (greenstuff) came with a sticky rubber backing pad which you were meant to attach to the back of the pad. I don't know if it was meant to replace the shims or not, so I just attached the rubber pad & assembled with the shims as normal. I did wonder if it was either overkill by the suppliers or whether the new materials were more prone to squeal ?

Brian

ps - how many miles since the re-build ? if it's all new, maybe a couple of heavy applications might bed everything in better ?

[Yum-yum]

Hi Phil,
We met last year up in RWC.
I don't believe the bores should be honed, I haven't done it for 2-3 brake services, and I don't believe the caliper halves should even normally be separated, I could be wrong on that.
My S4 is not much later than yours, I've never had brake squeal but saved this possible modification to the shim as a reference in case I ever did (see the attached sketch). I may have found this notice on these boards so a search may turn up more info. I think our front disc brakes come from the Tiumphs. You might consider this mod all else failing.
Alex

[oldelanman]

I believe the WSM requires 2 to 4 mils end float after which you back off a flat and insert the cotter pin. So technically, the front wheel bearings aren't preloaded.

That's going to be a bit slack .....I think you mean 2 - 4 thou (0.05 - 0.10 mm)

As for the brake squeal....putting a very slight champfer on the leading and trailing edges of the pad material can help reduce judder and may also work for squeal - might be worth a try if all else fails.

[Higs]

Should the piston bores be lighly honed when installing new pistons?
......
Is honing the piston bores recommended?
TIA

You do not need to hone the bores as the piston do not ride on the bores - they only, in theory, touch the rubber seal fitted inside the bore. Of course, the bore needs to be in reasonable condition to stop the piston catching but perfect condition is not critical.

Richard

[Galwaylotus]

I believe the WSM requires 2 to 4 mils end float after which you back off a flat and insert the cotter pin. So technically, the front wheel bearings aren't preloaded.

That's going to be a bit slack .....I think you mean 2 - 4 thou (0.05 - 0.10 mm)

As for the brake squeal....putting a very slight champfer on the leading and trailing edges of the pad material can help reduce judder and may also work for squeal - might be worth a try if all else fails.

In the US colloquial, mils means thou (milli-inches or thousandths of an inch)!

[oldelanman]

I believe the WSM requires 2 to 4 mils end float after which you back off a flat and insert the cotter pin. So technically, the front wheel bearings aren't preloaded.

That's going to be a bit slack .....I think you mean 2 - 4 thou (0.05 - 0.10 mm)

As for the brake squeal....putting a very slight champfer on the leading and trailing edges of the pad material can help reduce judder and may also work for squeal - might be worth a try if all else fails.

In the US colloquial, mils means thou (milli-inches or thousandths of an inch)!

Oops .... my apologies Rick. That shows how much I know Thanks for putting me right.

I'll make a note of that for future reference.........together with hood, trunk, header, runner, wrench etc etc

[AHM]

I have found in the past that you can get a residue where the seal sits on the bore of hydraulics, with brakes this can cause the piston not to return as it should and pads to wear/squeak

When rebuilding, I always clean the bores with very fine wet and dry, and polish pistons with metal polish, to ensure that the movement is smooth and even.

[pamitchell]

Thanks everyone

I took Alex's and Rogers suggestion on triming the anti-squeal springs & chamfering the pads brakes. I installed new seals & pistons & rotors last year. Before I road test the car, I have 1 question,
The S4 manual brake section (J) shows checking the gap between caliper & disc (Fig. 7 p. 11) How do you measure the gap and what is the measurement?
TIA

[UAB807F]

Hi,
The gap appears to be being measured with feeler gauges, presumably both top & bottom being checked. I've no idea what the measurement should be, presumably it's done both sides of the actual disc to ensure that it's central ?

All I ever do is clean up the mounting faces for the calipers & discs and then bolt everything together. If you do it correctly then the disc itself will run true (assuming it's not warped ) and the caliper will be central. I always spin the hub before putting in the brake pads to check for free clearance but that's about it. As long as it's not catching, the dimension is so small that I can't see what difference it would make other than one pad being slightly thinner ?

Brian

(the only other thing I've found myself doing recently is to check the pads move freely within the caliper. For some reason I've had a few where the backing plate has been too long and they've caught, so I've ground off a few thou' off the steel to allow free movement)

[oldelanman]

Perhaps the intention is to check that the gap between caliper and disc is parallel so that the caliper pistons are presented squarely to the pads and do not tend to tilt and jam in the bores.

I agree with Brian, if the caliper and disc mounting faces are clean you should not have any issues but I would check the disc for run-out after assembly just to make sure.

[Galwaylotus]

(the only other thing I've found myself doing recently is to check the pads move freely within the caliper. For some reason I've had a few where the backing plate has been too long and they've caught, so I've ground off a few thou' off the steel to allow free movement)

I think that's the problem I experienced recently where my LF brake was dragging and eventually boiled the brake fluid. On both calipers I had to drive out the pads as they were stuck in the caliper! I hadn't installed the pads myself but will next time so I'll ensure they move freely!!!

[mgourwcg]

I am also in the middle of fixing a dragging left front caliper as well. There could be another reason that has not been mentioned and that could be that the brake hose could have some foreign matter in it that is not allowing the brake fluid back up to the master cylinder as quick as it should

A question
Does any body know how the balance between the front and rear braking pressures is achieved?

Michael

Still running in the engine when I get the brakes to stop playing silly b's

[UAB807F]

A question
Does any body know how the balance between the front and rear braking pressures is achieved?

I think the hydraulic pressure through the system is the same, at least on a single circuit system. The braking difference between front & rear is probably down to the design/sizing of the calipers & discs. Smaller pads on the rear means less available friction area & I'd presumed lower overall efficiency even without weight transfer to the front tyres during braking ?