What I learned about brakes

[RotoFlexible]

In the process of sorting a soft brake pedal after upgrading to +2 front brakes on my S2, I learned some things I didn't know.

1. Lubrication for the pistons and seals is important. What I thought was stubborn air in the lines turned out to be sticky seals. (Thanks to our local guru for this hint.) Without lubrication, the seal grabs the piston and retracts it too far. I removed the pistons and coated the seals and pistons with caliper lube, reassembled and bled. End of soft pedal. I didn't use Castrol-Girling red rubber grease - hard to find here in reasonable sized containers - but the Permatex synthetic caliper grease I got should be fine.

2. A Mity Vac allows one-man bleeding, but only if you grease the bleeder threads.

3. I did the initial bleeding with the pistons fully retracted into the bores, then pumped each one out 1/4" or so, squeezed it back in, and bled again. This gets the last of the air out of the bores.

4. A Presta bicycle tube valve and a bicycle pump make a good tool for blowing pistons out of calipers, if you don't have compressed air or the correct fittings. Cut the valve out of an old tire and trim away most of the rubber, leaving a circle about the size of the banjo bolt. Push and screw the valve into the port. Hook up a floor pump, give it a couple of strokes, and stand back.

[SADLOTUS]

Cheers matey

[Foxie]

In the process of sorting a soft brake pedal after upgrading to +2 front brakes on my S2, I learned some things I didn't know.

1. Lubrication for the pistons and seals is important. Without lubrication, the seal grabs the piston and retracts it too far.

4. A Presta bicycle tube valve and a bicycle pump make a good tool for blowing pistons out of calipers,

Yes, you're dead right, I discovered this while trying to reduce pedal travel, and the older the seals the more they stick. Even with new seals and lube, they still pull back a little. I further reduced pull-back with helper springs (available from AP) under the pistons, I used the lighter type, with the heavier type the piston contacts the disc.

I made up a rig for removing pistons from a spare M/C and a cheap foot pump frame, never fails

[collins_dan]

The brakes on my S4 (non-servo, dual circuit MC) work great, but... the brake fluid is at least 4 years (when I bought it) and the MC is now starting to leak.... so I know I have to do it. I wasn't planning to replace the seals as they seem to be working fine, no leaks and brakes work great. I was planning to replace the rubber brakes lines with stainless steel when I do the job. I keep putting the job off as the brakes work so well, I'm afraid I'm going to screw them up and then head down one of the awful wormholes that I read posts about. A friend with a TR6 is going to help and the plan was rebuild the MC and install the new SS lines, then starting from the right rear, left rear, right front, left front (left hand drive car), bleed the system with him pumping the brakes and me bleeding. Since I'm not replacing the seals, do I need to lubricate them? Also, I'm not sure I understand what you mean by point 3. This is going to sound stupid, but when are the pistons fully retracted, when you foot is to the floor or when off the brakes. Thanks. Dan

[RotoFlexible]

To clarify: Before I bled the brakes for the first time, I pushed the pistons all the way into the bores, to minimize the amount of air in the caliper. I then bled them (with the Mity Vac) until major air bubbles stopped coming out. Then, without the pads in place, I pumped the brake pedal a few times to drive one of the pistons (one will be stickier than the other) out of its bore about 1/4". This gets fluid into the bore. I then pushed that piston back into the bore all the way (Channel Lock pliers work well) to (hopefully) drive out any air that's trapped in the bore. I put the pad in place to retain that piston, and repeated the procedure with the other piston. With both pistons done, I bled again, getting a little more air.

I did notice that one piston was still over-retracting. I "exercised" it by pumping it out and pushing it back one more time, and it was fine after that.

By the way, I found a good write-up on brake theory and overhaul here. It is oriented towards Triumphs with rear drum brakes but I found it very helpful, especially the description of dual MC's.

As far as your "don't fix what ain't broke" approach to the calipers, you should at least inspect the boots (around the ends of the pistons) to make sure they are still sound. If they are not, it is likely that the pistons and/or bores have some corrosion. Your PO may already have replaced the original chrome-plated pistons with stainless steel; if not, corrosion will eventually become an issue if it isn't already.

[GrUmPyBoDgEr]

This may be worth mentioning:-

Some while ago there was mention of the dangers of pushing the pistons fully back into the calipers.
If the pistons have corrosion or any other crud on them & that is transferred into the seal; the seal could get damaged & a leak could occur.

Cheers
John

[paddy]

If the discs and pads are newish, the pistons pretty much need to be fully retracted in any case.

Paddy

[AussieJohn]

I have been chasing a lowish brake pedal and today found that one pad on each front caliper retracts about 8 thou, I rebuilt the calipers with new pistons and seals, I thought the pistons were sticking and dismantled the calipers a few times but all looks good. Today I put a feeler gauge behind both the retracted pads and bingo, a good pedal. I am thinking of going down the " helper spring " route but as I have never gone down that way before I am at a crossroads; any comments or ideas?

[dusty]

helper springs (available from AP) under the pistons, I used the lighter type, with the heavier type the piston

Any part numbers or pictures for these Sean? Where did you source them?

Cheers
Jon

[Foxie]

helper springs (available from AP) under the pistons, I used the lighter type, with the heavier type the piston

Any part numbers or pictures for these Sean? Where did you source them?

Cheers
Jon

My filing system comes in handy from time to time

The 'anti knock back' springs are AP Racing products, supplied by Demon Tweeks. Part number for the 4lb spring is A/P CP2667-105. The 7lb spring is too strong, the pad was in constant contact with the disc.

See http://www.apracing.com/calipers/info.a ... prings_110

Willwood also supply a 2psi RPV (Residual Pressure Valve) which keeps a minimum pressure in the hydraulic system as an anti knock back measure. I think Citroen brake systems might have used this system

Sean

[Robbie693]

I had rollback on my new calipers I fitted a couple of years ago. On the advise from someone on the forum I exercised the pistons in and out a few times then wedged the brake pedal down for a couple of days with a piece of wood. Result was rollback vitually gone immediately and then cleared up completely after a few miles driving.

Hope this helps.

Robbie

[AussieJohn]

Been chasing my brake problem regarding rollback and have just tried the "exercising the seals" solution; best brake pedal for 2 years, havent driven it yet but very pleased.

[Rob_LaMoreaux]

One of the most frequent causes of knock back on disk brakes is the rotor being warped. So the rotor pushes on the pads causing the pistons to move back in the cylinder. Of course the car has to be moving for this to happen so if it happens sitting still the seal is sticking to the piston too much and the method mentioned above should help.

Rob

[paddy]

My experience is that in practice it's not due to runout of the disc, but the (necessary) end-float on the front bearings. For me, the pads can back off slightly if you steer from side to side on a straight road. If you just drive straight, it doesn't happen.

Paddy