ok, so I converted my 914 rear brakes to vented disc with 911 M calipers and internal drum parking brakes. Brakes work great but the pedal travel is longer than it should be. Went to rebleed everything and found that one of the rear calipers had a pad to disc clearance of .010" which probably explains the travel. Also verified that the pistons were working and not hanging up.

I've got a rebuilt proportioning valve from PMB that is working correctly. Running an ATE 19mm MC (bought a new one thinking it had the 17mm). No bubbles in the system using ATE SuperBlue brake fluid.

I've never seen a caliper piston release that far just on one side. Anyone have any ideas? This is frustrating!

Do you have the 5mm spacer between the caliper and the mounting bracket on the trailing arm ?? Are you sure they are M's ?? 84 on up 911's used a rear caliper the same size except the middle spacers were 4mm wider for 24mm wide rotor versus the earlier 20mm wide rotor the M's used. Some one else probably knows what else to check. Good luck.

Jaxdream

Thanks Jax. I do have the spacers in there. The calipers are about .090" from being perfectly centered. Going to see if I can get them exact. Problem is that one side works and the other side has the .010" gap. Both are set up with the same spacers.

check the rotor runout side to side. Maybe the rotor is pushing it out. Also I'm not aware that .010 is too much pad clearance but maybe so.

Is it returning that far, right after bleeding, or after a drive.

As I understand it, the 911 rear M's had an "anti-knock back" mechanism within the piston that was to prevent this, if there was excessive wobble of the rotor (due to the fact that you can't snug up the one piece rear bearing).

This would only effect you if the run-out (or wobble) of the rear rotor is excessive.

Wes

take the pads out push the pedal to push out pistons, then push the piston back just enough to get the pad in instant higher pedal.

New rebuilds sometimes the piston does not stay out.The resilient new seals pull em back too far( rubber band)

" (due to the fact that you can't snug up the one piece rear bearing)."

WHAT?

The Cap'n

It returns that far just sitting in the driveway pushing on the pedal multiple times and releasing. The wobble is ruled out.

Most of the early calipers have the "anti-knockbacks" in them. Knockback or not, they shouldn't be releasing this far.

I had an extra .050" shim on the calipers that I took out but that didn't solve anything.

I'm down to:

A. Tearing apart the calipers, honing the bores, polishing the pistons, putting it all back together AGAIN or buying a pair of rebuilt calipers.

B. Something weird going on with the proportioning valve where it is pulling back too much fluid. I doubt this though because it can only pull back as much fluid as I push through it.

C. Something weird going on with the master cylinder.

D. All of the above

ya

As was stated earlier, this is not uncommon with fresh rebuilds. The best solution is to drive it a few hundred miles around town to get the seals 'worn in'. Then check it again. The knock-back effect of the seals is quite pronounced after rebuilding. Were the pistons & seals put in dry, or were they lubricated during assembly. I use special lube for calipers during assembly, and rarely have the problem.

Thanks Dave. I didn't rebuild them myself or they would have been lubed for sure!

When I took out the .050" shim, the inside pad is now against the rotor somewhat hanging it up. I haven't driven it yet so maybe putting the wheel back on and taking it out will solve that.

As for centering the caliper, what is the general consensus? Should the caliper split be centered over the midpoint of the rotor OR should the pad "guides" cast into the caliper have the same distance from the face of each side of the rotor?

ok, maybe a picture will help.

Should the caliper be centered on the rotor using the split between the caliper halves (RED arrow)

or

Should the "pad guides" cast into the caliper have the same spacing away from the rotor face on both sides of the caliper (YELLOW arrow)

or

It really doesn't matter as long as the caliper isn't rubbing against the disc and it's close to center...

Hey dave p , what is the name brand and or part number of that special lube ??My FLAPS don't carry any caliper assembly lube ,small town, may have to order / go to bigger town to get. I've seen a special lube on the S caliper thread.

Jaxdream

Permatex Synthetic Brake Caliper Grease. Comes either in small packets or a tube.

I'd stay away from the Silicone stuff.

Synthetic Brake Caliper Grease

Tom,
If it comes to it, we'll just grab the brakes off my dad's 911s on Saturday, and try them... . he'll never notice the missing alloy calipers.

Otherwise, BUMP to get your caliper centering questions answered, perhaps.

It's all good. They should be ready by Saturday.

The "pad guides" cast into the caliper should have the same spacing away from the rotor face on both sides of the caliper (YELLOW arrow).

The lube that I use was a VW part that was dropped 10 or 20 years ago called Brake assembly paste. Probably from ATE. Since it is a big as a toothpaste tube, it has lasted a hundred or more caliper rebuilds.

Thanks Dave. That's what made the most sense to me. Now to find four .030" shims!

BTW, ripped apart one of the supposed "rebuilt" calipers to find rusty cylinders and pistons. Got one done, the other to go.

Thaks , davep. I goggled for brake assembly lube , from what I read ( ??) it seems that ATE brake paste is not allowed to be inported for evironmental / msds stuff , so I search on. Thanks again.
tomeric914 , it sounds like your caliper pistons are sticking , and the rebuild should fix that , as for the .030 shims ,thats not a lot and would probably be offset by the caliper rebuild, but hey , if ya gotta do it ,do it right. Yopu can get shim stock ( stainless , brass ,etc ) by the roll off of ebay fairly cheap , or maybe Napa carrys shim stock . Good luk

Jaxdream

Thanks Jax, they were sticking and are now all nice and rebuilt. I ordered 5mm fixturing washers from Grainger back in September that turned out to only be 4mm. Added in a .050" shim to get it right.

Still too much pedal travel and the rears are locking before the fronts. Bled a few times now with good flow. I suspect the MC but hate to put it in if it isn't the problem. Brake setup on my 914 is essentially mid 70's 911 except that is has the 914 proportioning valve. I'm going to bleed the rears one more time. If I still have the same problem then I'll change the MC.