Brake Troubles

[SGomes]

Let me introduce myself. My name is Shannon and I've been a lurker here for a few years.

I have recently upgraded my front calipers to BMW320i variety.
I have recently upgraded my brake lines to stainless braid variety.
I have recently replaced all four rotors and brake pads.
and yes, I had someone else install a 19mm master cylinder.

Here is my problem... I have followed the tech articles on here for everything that applied to this job from replacing the pads to bleeding the brakes. But I have NEVER been able to get the darn thing to be anything but spongy...and by spongy I mean I can push the pedal all the way to the floor if I push hard enough. Also the brakes drag on all four wheels. It's like they will clamp down when you press the pedal but they don't release all the way. On the rear brakes I've adjusted the venting clearance about a million times. On the front brakes there is no adjustment so they just drag all the time.

Any suggestions??? I'm at the end of my rope on this. Could it be the brand new 19mm master cylinder? What am I doing wrong? Oh yeah I just recently purchased the pressure bleeder that is sold here. (makes the job way easier) but didn't help a bit with my problem.

[SGomes]

...and just like a typical newbie I didn't look deep enough into the archives.

I just found a post from about six years ago about the very same problem. It looks like the solution MIGHT have been to check the clearance on the MC push rod. Perhaps the MC can't release far enough... Now I just have to figure out how to check and adjust this clearance... (I had someone else do the MC replacement)

Does this sound like I might be on the right track? Can I check this adjustment from on top or do I have to get under this beast to check it?

[Dave at Pelican Parts]

The adjustment is on the pushrod, inside the cockpit. It could help, but I'm not sure it will.

You can also bleed the proportioning valve--it is kinda tough to get all of the air out of it. It can also be difficult to get all of the air out of the master cylinder.

--DD

[lmcchesney]

Shannon it seems that you have done a great effort in making your 914's braking system up to date.
First for the grinding of the front 320i calipers. Assure that the surface of the mounting flange has been milled 0.125" to allow center of rotor to center of caliper alignment. Since you replaced the rotors, you have both alignments to assure.
As DD has said, the proportioning valve can accumulate and retain air. Because of its flat location, removal of the air is problematic. A few other suggestions for bleeding the brake lines.
To make the bleeding process less messy, I found a simple device at my local auto parts store. It is simply a small containment bottle with an attached flexible tubing to connect to the bleeding nipple to contain the bleed fluid.
Using the combined family member/pressure system. This requires understanding by the person compressing the brake pedal that at no time can they lift the pedal while the bleeding nipple is open. Pumping up" the pedal allows a larger volume of brake fluid to be flushed through under hydraulic pressure. It takes time and cooperation.

Follow the bleeding sequence: right rear, left rear, right front then left front.

Another technique I have used in the past is back bleeding. Using the above containment plastic bottle filled with brake fluid, back flush the proportioning valve from left to right.

Hope this helps.
God's Blessings.
L. McChesney

[tat2dphreak]

what dave said... plus tax! I had the spongy brakes until I bled that little b*st@rd

[SGomes]

Thanks guys! This board is a wealth of knowledge!

Interesting thing to note: When I left the car yesterday the brakes were locked up on all four corners. I went out there this morning and they were still firmly locked up! This sounds like good news to me! If I had a leak or an MC problem it probably would have relaxed over night.

Thanks for mentioning the clearance issues with the 320i calipers. Yes mine had the 0.125" milling done to them before I put them on.

Looks like I'm going after the PValve first. Some tech article I read somewhere suggested cracking open the output of the PValve and having a family member push gently on the pedal. Sounds like it will evenly distribute brake fluid all over me and the engine compartment but will it help get the air out?

I'm still going to look at the MC push rod clearance but only after I bleed that PValve.

[Dave at Pelican Parts]

Remember, only the later cars (73+, probably some 72s) need to modify the BMW calipers. Early cars should not need that.

L. McChesney, you were precisely describing the power bleeder that SGomes mentioned in his first note.

SGomes, cracking open the output of the P-valve is indeed a good way to get brake fluid all over everything. And brake fluid eats paint. (Slowly.) So you probably want to have a rag loosely wrapped around the fitting when you are opening it up. And make sure to wash down the area with water afterward--water actually dissolves/dilutes brake fluid.

That "brakes stay locked up" problem doesn't sound so good. It could be something as simple as the calipers themselves sticking... Were the BMW calipers rebuilt, or used? Have the rear calipers ever been rebuilt?

--DD

[TheAndante]

Shannon, #1 would be check the adjustment on the mc plunger rod. It sounds like with the pedal at rest, the rod is partially engaged and in turn the brakes are partially engaged.

As Dave asked, have any of your calipers been rebuilt? If they have been rebuilt what was actually done to them? I've compared brakes to a parachute. You wouldn't take a 20-30 year old chute out of an old chest and go jump with it I hope. If they were not split open and totally gone through and all perishables replaced then you have to put the word "rebuilt" in quotes.

If you haven't checked by now, make sure that your rotor is passing right down the center split on the front calipers. Though not a solution, individual things can add up. Since it sounds like the problem is shared by all 4 calipers the mc and the p-valve are prime suspects.

Concerning bleeding, if you pump the brake pedal ala the two-man method, I really feel that thrusting the pedal down sharply helps to ensure that trapped air is forced along the lines instead of it being able easily creep back, against the flow. That assumes repeated thrusts. Good quick thrusts can also help move and dislodge trapped air.

Paul

[SGomes]

Dave, Andante thanks for the info.

A rag around the PV is surely a good idea. My plan is to crack it open a bit (enough to see some fluid leaking out. Then have the family member push SLOWLY on the pedal to push out a fair amount of fluid. While they still have their foot on the pedal, I'll tighten it off so as to not let it suck air back in. Then I'll go take a shower.

I'm not too worried about all four calipers needing a rebuild at the same time. The BMW's were pretty damn new when I got them. The rear's well...ok they are a bit crusty. I don't know their history and to be quite frank I'm worried about one of the pistons in one of them. BUT I'd like to get the system working first, then I can tell if just one of them is sticking.

Concerning pushing on the pedal... yeah I'm in favor of stomping on it pretty good. As I understand it, the PV needs a bit of pressure to get it to open so I say go for it.

I still don't know how in the heck to check the MC plunger rod clearance. My haynes manual should be here in a couple of days. Hopefully it will have some notes on it. Anyone know of an online resource?

[SGomes]

Imagine my surprise when I go to look at the MC push rod adjustment and notice the portion of the MC I can see is quite rusty! ... hmmmm

THE F&%(&^(*ING MECHANIC DIDN'T PUT IN MY NEW 19MM MASTER CYLINDER!!!!

I am just so sick and tired of lying, cheating, worthless mechanics!!! Why does it have to be so hard to find someone who isn't out to get you?!?!? I mean...ok so they got some money out of me for the one job but they have to know that I'd never go back for return business! They're missing out on the bigger picture. Is it really so hard to just do the damn job right????

[tat2dphreak]

peaking for someone who put in a MC... it sux, I wouldn't do it for a living, no way, no how... took me over an hour the first time!!

[TheAndante]

Shannon I must say that if I was at this same juncture, and if I had a choice of mc's(as in not already totally committed to a 19mm), I would opt for the OE 17mm, even with the BMW upgrades in front. Based on my experience and the basic physics of it, it has always appeared to me that the 19mm requires more pedal pressure to accomplish what the 17mm is doing already. (assumes good working mc's)

What the 19mm does contribute is it moves a greater volume of brake fluid, which is not something you benefit from with your setup(assuming street use only), even with the BMW calipers. There is a 23mm Mercedes mc that will also bolt up(with modification to the ears?) to the 914. A 23mm would require even more pedal pressure and move an even greater volume of brake fluid. Is the difference in pedal effort between the 17mm and the 19mm perceivable to everyone? Maybe not. But if just the price of the 19mm is higher I couldn't see getting one here.

Does anyone have more input on this? This is a perception based on numbers I've seen and my own experience. Are there other detriments with the 17mm or benefits with the 19mm on the brake setup above?

Paul

[SGomes]

Harvey Hoover did an interesting study of this on this site:

http://www.pelicanparts.com/techarticles/914_BMW_calipers/brake_calc2.htm

Basically, from all the discussion this subject has gotten on this site, it seems to come down to "feel". I have no idea if the 19mm is going to "feel" better but I know right now I don't like the "feel" of pushing the pedal to the floor!

I do know that I will "feel" a whole lot better being behind the wheel rather than under them when I get this car back on the road!

[Scooter311]

Howdy, and welcome!
I had to pull my fronts off, pop out the pistons, and give 'em a healthy scrubbing. New seals from our host, a 10 minute bleed, and no more mushpedal. Mind you, this was after I realized I had them on upside down, and had been bleeding them for most of my adult life. You mention one of the rears is crusty and questionable (theres a bad joke in there somewhere..) - It's probably a good idea to pop the pistons out and put in new seals, that was really what my problem was after all was said and done.
Good luck, and good bleeding
Jeff

[Dave at Pelican Parts]

Originally posted by TheAndante
Is the difference in pedal effort between the 17mm and the 19mm perceivable to everyone? Maybe not. But if just the price of the 19mm is higher I couldn't see getting one here.

Does anyone have more input on this?

Yup, I do.

I think you could notice the difference in a back-to-back comparison of systems that are in good working order. There should be about a 25% difference in the pedal effort for a given clamping pressure. [(19*19)/(17*17) ~= 1.25]

The 19mm MC is either the same price as, or less expensive than, the 17mm one. (It changes from time to time, but they always seem to be close in price.)

The 19mm one has about 25% more cylinder bore area than the 17mm one--but the BMW calipers have about 25% more cylinder bore area than the stock 914 ones! So the 19mm MC is actually a decent match for them. Yes, this ignores the rear calipers, so the total system difference will be smaller... But not in an unreasonable range, IMHO.

I've driven 914s with stock brakes and the 17mm MC, and those with stock brakes and the 19mm MC. I'm not sure both systems were in equal working condition, and it was not back to back, but I found there to be very little if any practical difference. The pedal felt a little harder, I think, on the 19mm one, and I may have had to work my leg a little harder. It wasn't an issue, for me.

--DD

[lmcchesney]

Shannon,
If your MC plunger is rusted you don't have much choice. Your decision is not whether you to replace/rebuild the MC but 23 vs 19 vs 17mm. Which one is not that critical. Even the experience of DD supports that. If you will feel better talking to your peers that, "Yeah, I went to 19mm MC", or you like the physics presented by Harry Hoover and wish to go with 17mm MC. As Dave said, you can't compare side to side. Neither functioning MC and system will result in pedal to the floor and both will provide a firm system.
The best mechanic long term for you will be yourself. Having any 30 year old car, part of the enjoyment in having one is the puttering arround with making something better. If you find or have found that the repair aspects do not provide enjoyment, exchange the 914 for a recent model and enjoy the driving.
L. McChesney

[SGomes]

...lauging hysterically. Yeah...I don't think any of us who own a 914 expect to just drive the bugger every day. There is a certain amout of wrenching that is enjoyable....and inevitable!

I'll let ya'll know what happens after I get my new MC in and re-bleed the brakes for about the millionth time.

[TheAndante]

Originally posted by Dave at Pelican Parts


I think you could notice the difference in a back-to-back comparison of systems that are in good working order.

Precisely! But again could everyone perceive the difference? Maybe not. As Shannon said, it does seem to come down to "feel". For me, less pedal effort is a big deal and very noticeable. To the next guy it may not be of any consequence, perceivable or not. All 3 properly working mc's will lock your brakes. Now if one were to do rear BMW upgrades as well, then the 19mm would be an absolute must.

Paul

[SGomes]

Well, tat2dphreak, I'm impressed!! You said it took you a whole hour the first time....I've been at it for 3 and I still can't figure out how to do it!

I looked at the pelican tech article and there is 3 or 4 suggestions on how to get those darn feed lines in.... so far I haven't been able to do it. I got one but the second is no go.

Here is my question:

I've detached the feed lines from the resevoir. I'm going to replace the rubber hoses anyway... I'm trying to pull the lines down so that I have more to play with under the car but they don't seem to want to move any more than a couple of inches. Is there something I can't see that is holding them? I'm fully prepared to remove the gas tank if I have to.

[tat2dphreak]

you aren't missing anything and I don't think removing the gas tank will help... it's a ***** plain and simple... I jaacked the car up further for more leverage... I also put the lines in the grommets 2-3 times and removed them to open up the grommets... when they would go on easier, I put the m/c on the pegs (BARELY enough to get 1 of the nuts started) and put the lines into the grommets, then tightened it down(snug but not tight, where it could still move a little) and put the brake lines in and then tightened the bolts... I have to do this again soon and I'm not looking forward to it... but once the grommets are opened up and looser, it makes it a little easier