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

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!

I have this job in the near future, so many thanks for the details.
Now go out enjoy that teener!

OK thanks... good luck. Let me know if you have any issues, questions or need pics.

Glad to hear that you have your brake system bled. Just have to say that your recomendation to remove the brake force limiter (proportioning valve) is NOT a good idea at all. Here is some information from the folks at Porsche, who designed the car:

"The rear wheel brake circuit includes a brake force limiter. When pressure in the rear brake circuit reaches a predetermined level (changeover pressure), the limiter prevents the rear wheel brake force from increasing further at the same rate as the front wheel brake force. In this way rear wheel locking is prevented as weight is transferred to the front of the car when braking"


This device is an important part of our brake system, an early sort of ABS. I don't think it is a good idea at all to remove this part, and it is a major mistake to advise others to do so.

Again, I am well aware of the controversy... and well aware of your quote. However, after talking to many experts on these cars who have decades of experience working on them and racing them, they all agree there is no data to support leaving it in. It's a personal preference and it was the only solution that worked for me. I'm not recommending to remove it unnecessarily, and I agree that if it's installed, operational and there are no issues, by all means leave it in! If I could buy a new one, I would absolutely give it a try, but as you know new ones aren't being made. So I guess maybe Porsche shouldn't make those statements if they don't plan on supplying the parts to support their cars? Just a thought!

Pmb supports our cars by rebuilding the proportioning valve to new spec.

We've seen this topic of the PV come up many times.

If its operation is complementary to "load" transfer under hard braking then adjustment is necessary to complement modern suspension upgrades.

I've prepared my car specifically for autocross and I can tell you the transfer is considerably less than stock. So mathematically the PV should be adjusted to suit.

That being said, I've never locked up a rear...

Air can also be expelled in the rear calipers by working the parking brake while pumping the pedal, then pressure on the pedal while the bleeders are open in the usual bleeding method. The mechanism actuates everything inside the caliper, releasing air from all of the complex parts inside as they move and getting you a better pedal. Just another idea from experience...

Hah! Great tip! Could have used that tidbit a few gallons ago!

Exactly!

Most forget to bleed the proportion valve, it is the highest brake part on the car, using the power bleeder makes it easy, put the system under pressure, place a rag over the pv, crack each of the fittings, air gone.
Also you should bleed upper and lower bleeder valves on the calipers, bottom first.
Most problems with pedal travel are related to the venting clearance.
Mike

Yep...I did it this way years ago at Eric's suggestion.

No issues. Sorry about your 5 months of sorrow.

I also disagree with your statement of removing the PV valve and lack of data.

I notion the engineers knew and had plenty of data to support its use and design.

Opinion is something else.

Post your pics.

thanks for the tips and writeup.

questions:
1. what is the technique for bench bleeding the calipers?
2. what is the venting clearance and how do you measure it?

I wouldn't drive a 914 on the street with a "T" fitting in place of the PV.

Has anyone had experience with an insurance company dismissing a claim due to a modified braking system?

1. One way to do it is to suck brake fluid into the calipers using a vacuum pump, or use a hand pump to force it in. These calipers apparently have a bad habit of trapping air bubbles so rolling the caliper around a few times and pumping/sucking more fluid before bolting them on should theoretically help get rid of the trapped air. Then bleed per the recommendations found in many threads in this forum.

2. The venting clearance is the space between the pads (inner and outer on each rear caliper) and the rotor. The gap, according to PMB should be about .004". I found on mine I needed closer to .001" Hard to believe a few thousandths can make such a big difference on pedal travel. I'd start with .004 then go from there if necessary. Simply slide a feeler gauge between the pad and rotor and adjust the gap with the adjusters. The inner pad adjusters can be a little tricky but once you get the cap off and fell your way into the allen head, it's pretty easy.

I have a rebuilt PV from Eric on my '73 with "new" calipers also from Eric.
I've bled ( and done the trick for the seals as per Eric) till I'm blue in the face and though the pedal is "ok" it is not firm or as rock solid as my '71 with OEM 914-6 brakes F and R.

Guess I'll try that trick on the rear nipple and fiddle with the parking brake... already done the venting clearance, but will check AGAIN!!

I feel your pain. If you can't get pedal from the bleeding trick, to troubleshoot, I would replace the PV with a "T" and see if you get pedal... that way you will know if there is air trapped in the PV, assuming it is operating correctly. Also, take the venting clearance down to .001" and see if that helps.