Late 914 2.0 Brake Bleed Nightmare, Finally a resolution -Lesson learned Options

[flypfi]

I am posting this to hopefully save someone a lot of time and aggravation when bleeding the 914 brake system. So, here is the history... The car is a 1976 914-4 2.0 with dual nipple calipers. The right rear caliper was seized and dragging, the pedal was soft and braking was marginal... so I thought it was time to give the brakes some serious attention. I replaced the MC with a stock 17mm. I replaced all 4 flex lines. I removed the rear calipers and had them overhauled by PMB (the fronts appeared to be in great condition - pads and rotors new). When I got the calipers back, I installed them with new pads and rotors, set the venting clearance to .004" and started to bleed. As an important side note, there was no mention in the literature from PMB about pre-bleeding the calipers before installation so I did not pre-bleed them. And here is where the odyssey begins. I've been bleeding brakes for 40 years and never had a nightmare like this! I used a power bleeder, then the power bleeder with standard pump and hold... then tried with speed bleeders, bled the proportioning valve several times... back to power bleeding.. etc, etc. Never could get more than 1/2" of pedal.

Then, after reading about other's experiences, I removed the proportioning valve and installed a bleed nipple on the line from the MC leading to the valve (eliminating the rear calipers and the valve). Bled that line once and had a rock solid pedal with about an inch of travel, so I knew the front brakes and new MC were good. Then I removed the PV and went directly from the MC to the rear calipers. Bled and bled and bled and bled! Several gallons of brake fluid later, I had a pedal that is good enough to bring the car to a stop, but not securely. The pedal travels about 2/3 down before the brakes engage and it is spongy as hell. At this point I had enough pedal to follow PMB's pad bedding procedure. Made no difference. So I figured there must still be air in the new rear calipers. Spent 4 phone calls with Eric at PMB and he basically said, "Yeah 914 brakes are a bitch. Air hides in several places." Then I tried hyper extending the pistons to relocate the new piston-seal alignment per Eric's suggestion which seemed to help a tiny bit, but really not sure if it was my imagination or not!

So, I continued to bleed the rears, using every frigging goofy suggestion I could find in the darkest corners of the web to no avail... they still sucked! Bleeding top nipples, bottom nipples (depending on what you read, there are several opinions on this) - made no difference. I was convinced there is air hiding in the rear calipers but I had no way to prove it, nor could I come up with a way to displace it without removing them and bench-bleeding.

So yesterday (been working this nightmare on and off for 5 months), I got an idea to expel the air by pumping fluid into the bottom nipple and opening the top nipple to push any trapped air through the caliper from the bottom to the top and out the upper nipple. Well... it worked like a charm. Good solid pedal and brakes feel great (for a 914 that is). I have been twisting wrenches for most of my life and have replaced a lot of wheel cylinders and calipers over the years and have never bench bled any of them - and never had an issue. Don't make this mistake on these brakes! Lesson learned.

Summary... here are the most significant take-aways from this experience:

1. If you are installing overhauled calipers... PRE-BLEED THEM ON THE BENCH!! In my opinion, PMB should make a big deal out of this when they return your overhauled calipers.

2. The venting clearance is critical on the rears. I found .004" too much. I went with about .001" which seemed to help. I noticed when I torqued the wheels back on after adjusting the clearance, it tightened the clearance slightly and the pads were dragging. This happened on both wheels, so if you have this issue, you have to anticipate a thousandth or so after tightening the lug nuts. I don't know if this is common or I'm just lucky (IMG:style_emoticons/default/mad.gif)

3. If you have the later style 2 nipple calipers, and you neglected to bench bleed (like me) try pumping brake fluid through the bottom nipple out through the top. I used a new, clean hand pump oil can with brake fluid and a clear tube. Make sure to pump all the air out of the tube before you connect it to the bleed nipple so you don't pump more air into the caliper. This technique cured all of my problems.

4. Remove the proportioning valve and replace it with a "T". I know this is controversial, but I had lengthy discussions with a highly respected Porsche mechanic who also races 914s and he claims the valve must go. I tend to agree after doing quite a bit of research and listening to both sides of the argument.

Pictures available on request. Hope this helps!