Brake bleeding and upgrade

[sundaypunch]

I just got through upgrading my brakes. Fixing a stuck caliper turned into rebuilt calipers w/ new pistons, stainless steel lines, some new hard lines, new master cylinder, and ATE super blue fluid. I also bought a Motive power bleeder. Everything is back together with no leaks, I just have to finish bleeding the system.

I'm having a hard time getting a firm pedal even with the power bleeder. It may be that I just haven't done it enough times. One other problem may be that I forgot to bench bleed the master cylinder. In any case, I'm using the power bleeder at 10-12 psi and it appears to be working perfectly. I've gone around the car (RR, LR, RF, LF) 5 times and the pedal is still spongier than I'd like. I've got to go down about 70% for full pressure. After one pump it's at the 30% travel it's susposed to be. After the first couple of rounds of bleeding I got no bubbles. I'm opening the bleeder for about 8-10 seconds.

Any suggestions? I'm not sure if I need to increase the psi on the Motive, try a pedal bleed, or just keep trying a few more times.

Thanks in advance.

[vash]

haha, exactly what happened to me. i got my girlfriend involved at this time. i used a wooded dowel and tapped hard turns in the line. she pumped the brake pedal and i went around. still spongy. it was suggested by someone to increase the psi to 20 and i went around two more times. presto, good brakes. i used about 3 cans of ATE after all said and done. good job, dont give up. i bet you just have to sacrifice more fluid. i didnt bench bleed the MC either.

[sundaypunch]

I went through two cans of the ATE. I thought that would be enough but no luck. I should have tried the higher psi, I didn't want to go much higher than the recommended 10 or so. The one area I think the Motive might be suspect (especially in an empty refill) is that the 10-12 psi must be much lower than what the system would see beyond the MC when you pump the brakes. I guess the low psi is just so that you don't pressurize it too much between the reservoir and MC? I did tap on the calipers with a rubber mallet but that didn't seem to dislodge any bubbles. I will try to tap on the lines as well.

I also considered the "Speed Bleeders" when I ordered my parts. I may give them a try even though they shouldn't be needed. I need to order more ATE anyway since none is available locally. They seem to have a good rep. in other threads.

[Early_S_Man]

I doubt that any futher brake fluid $$$ loss by bleeding will fix the problem. Check your pads to see how tight they are ... or more to the point, how tight the pistons are against the pads!

Check this thread to see how to firm up the pedal by reducing pad/piston clearance:

Brakes

[ischmitz]

There is a simple reason and a simple trick: When you replace the seal in the brake caliper and push the piston in all the way you won't get a hard pedal when bleeding. Ask me how I know. What happens is that every time you pump the pedal the piston pushes the pads to the disk but as soon as you release the pedal the piston retracts too far into the caliper. The new seal almost acts like a return spring. You need to carefully pump each piston out a little bit without the pad installed then push it back just enough that you have a hard time getting the pad in and then bleed the brakes. What a difference.....

Ingo

[ischmitz]

Warren,

My old M calipers had this mysterious thing in the center that looked like some adjustment or support to not allow the piston to retract. Forgot what you called it. However, my Carrera calipers are plain emty inside. No such thing. In any case a new seal needs to get broken in. At first it acts like a spring since it deforms a little bit when the piston moves. Pumping out the calipers makes a huge difference and is done in no time.

Ingo

[Early_S_Man]

Ingo,

The mechanism with the center post was called a deflection compensator ... I am not sure whether it was in the Clymer 912 Handbook, or one of the early Bentley VW manuals that described them in detail ... I know there isn't anything in the 911 service manual about them.

I first thought the deflection compensators were part of the problem, but SCs and Carreras seem to continue to have the same problem without them ... so I guess it is the new rectangular cross-section seals that are causing it.

[sundaypunch]

Excellent advice and link. This sure beats the trial and error of trying to figure things out in pre-internet times.

I did new pistons and transferred the "guts" under the snap ring from the old pistons. These were nice and clean. My wheels do spin freely but I can hear a very slight drag which appeared to be from the pads barely touching the rotors. I will try bleeding the next time around at 20 psi with a spacer in place of one one the pads. This little setback doesn't even bother me as I'm mostly thrilled to have no leaks after disturbing almost every connection in the brake system. Also, after the one pump my brakes took me from 80 to 0 in almost no time and didn't pull

Edit: I wonder if the Speed Bleeders would be successful without having to use the spacer in place of a pad? I don't think that 10 psi with the Motive is enough pressure to really seat the pistons against the rotors. It would seem like the extra pressure of pumping the pedal might help?

[ischmitz]

Warren,

exactly, the new seal deforms while the piston slides by them. Now everyone including me pushes the pistons all the way into the caliper upon reassembly. And that's where the problem start. Even lubricating the seals generously with brake fluid does not avoid that they'll retract the piston back into the caliper. You can watch that. Pumping out the pistons uses this spring effect to your advantage. Once the calipers get heat-cycled a couple of times this effect seems to go away. I guess you establish a wear pattern on the seal.

If you don't use the pump-out trick you might as well buy ATE shares. You'll go through gallons of ATE before you realize what's going on.....
Ingo

[spinner]

Whenever you open up the brake lines, you will get air into the system. You will need to flush out the system couple of times. Use cheap fluid for this - no sense wasting good ATE fluid. You need to bang the calipers around. Drive the car around the neighborhood and to the corner store (if you have one). Work the brake pedal. Flush again. You should get back a firm pedal. Now flush one more time with the good stuff like ATE. I have to go through this process whenever I open up the lines. It works everytime.

[sundaypunch]

I will bleed the system with cheap fluid using the spacer method to get a firm pedal. When I re-bleed with the ATE blue do I need to remove the pads again or can I just leave them? I'm not sure if I would be right back where I am now if I left the pads in.

[ischmitz]

If you pump the pistons out without the pad installed or with a much thinner spacer installed you should be fine. The trick is to push the piston in just enough so that you sort of have to wedge the pad in there to get it installed. That's preload the piston and prevent it from retacting too far every time you release the brake pedal.

I didn't use spacers. You have to do one at a time. I was watching the piston while a second guy carefully depressed the brake pedal. As soon as the piston came out far enough so that the pad would not simply slide into the caliper I used a big screwdriver to carefully push it in again while putting the pad in place. Once the pad got started I had to tap the pad so that it would slide in. No need to use different cheaper brake fluid here. You can get all four calipers bled and a hard pedal with about half a can of ATE if you get the hang of it.

Ingo

[sundaypunch]

Well, that was exactly my problem. The Motive was working perfectly and bled the lines in one or two passes. I had too much pedal travel because of the gap between the pistons and pads. There was about 1/8" at each pad and piston. The large travel I had was just this gap being closed. When I adusted this and re-bled the pedal is rock hard at about 20% travel. I did use 15 psi on the Motive to help things out.

To push the pistons out I cut a piece of plywood that was thinner than the pad. I pumped the brakes until this was tight and then pushed the piston back slightly so that the pads would have an interference when reinstalled. It only took one round of bleeding and the pedal is great.

Thanks again to all for the excellent info.