Brake Troubles Part II

[SGomes]

Well, one master cylinder later and I'm no better off.

Project to date:

Freshly rebuilt 320i calipers on the front
Freshly rebuilt calipers on the rear
Just bought 19mm MC
Stainless braided lines all around
New rotors all around

Now when I say "freshly" rebuilt...they were fresh when I put them on the car two years ago. Haven't driven it more than 10 miles since then.

Symptom: Brakes drag on all four corners.

The new MC made the pedal stiffer so that's good. I can move the pistons on the rear calipers fairly easily. I reach in there with my claw hammer and without prying I can pull the outside piston back. The inside one I pry with a flat bladed screwdriver. All four pistons move. On the front, I haven't tried moving them manually. They lockup the disks when I have my foot on the pedal and when you let go they relax and you can turn the wheel.

My bleeding method has been to pressure bleed the system to flush it. I followed the correct order: RR, LR, RF, LF. Then I follow this with the family member technique. A few pumps leaving the foot down on the last one. I crack the bleed nipple untill the flow stops then retighten it. THEN the family member lets go of the pedal. Repeat 3 times on each corner. Repeat all four corners. Over and over and over.

The pedal isn't getting any harder. And all four brakes drag just as much now as ever. Oh yes, I've adjusted the venting clearance many times.

I think I need to give up and have the car flat bedded to a shop.

On the plus side, if I have to rebuild the calipers again, there will be a '73 914-4 up on the boards for sale.

[mike95125]

Where are you located?

[SGomes]

I'm near San Jose, CA.

Other thoughts I had today:

A) I'm not getting enough sleep...oh wait different problem..
B) I'm pulling the pads out of one of the front calipers and see how stiff the pistons are. Maybe sitting for a couple of years has frozen them solid.
C) Just 'cause it was so much fun the last time, I'm going to rebleed the Pvalve. Do I really need that thing? I've heard of people replacing it with a Tee after upgrading to 320i calipers in the front
D) I don't need to flat bed it to the shop, I can remove all the pads and tow it myself! yeah!

[tat2dphreak]

you should replace with a "T" after the 320 brakes... it will save sanity too...

I do think you are on the right track with checking the front calipers... if they've sat for 2 years... they may be BAD... if they are machined for a73+, I would try to salvage them...take them off, soak them in brake fluid(not brake cleaner), then get a "C" clamp to push the pistons in, and use air to push them out again... if you can get a rebuild kit, I would... brakes don't like to set up... ask me how I know...

[Lyle]

When you replaced your master cylinder, did you check the length adjustment on the rod that goes from the pedal cluster to the MC? You do need some clearance so that the master cylinder can return all the way back to its home position. If it doesn't, there will be a small residual brake pressure, all of the time and of course that would cause your brakes to drag. When you bleed the brakes, you might be able to get rid of this residual pressure but it would return once you have used the brakes again. There is also one more thing that can cause a residual brake pressure, if you still have the "original" plastic bushings in your pedal cluster have been exposed to brake fluid, they will swell which in turn will cause your brake pedal to not return all the way to its home position. This in turn will cause residual brake pressure.
Reach down under the dash and pull back on the brake pedal. If it moves further back when you pull on it, then it is not in the proper home position. It should move all the way back to the stop on its own AND there should be a small free pedal play when you push it forward (roughly 12 mm). It seems to me that only a master cylinder related problem can explain dragging brakes on all four corners. The brake porportioning valve only affects the rear brakes and your 320i brake caliper upgrade only affects the front brakes, a master cylinder problem, (for example residual brake pressue) affects all of the brakes. It is even possible that the master cylinder itself is bad and the pistons are not or cannot return to the proper home position. Was the 19 mm new or rebuilt? Food for thought, goodluck
Lyle
Good luck

[srb7f]

Check your pedal play and bushings first, but what exactly do you mean by dragging? If you've got brand new pads on there and haven't driven the car, they will drag slightly until you drive the car and get the pads seated in properly. I've been told that it's a slight wobble in the disc itself that is responsible for backing off the pads, which requires that the car be moving to occur. If you've got a semi-stiff pedal (as good as a 914 gets), no air in the system, and can't turn the wheels when someone's on the brake, I'd CAREFULLY drive the car in a safe location and test them out since the pads won't grip at full strength till you break them in.

[SGomes]

Thanks for the replies!!!

Lyle:

I like your logic. It does seem that to make all four corners drag (all EIGHT pads!) that it has to be something systemic. It would seem quite unlikely to be EIGHT sticking pistons!

However, I did check the clearance on the MC after I replaced it. 12mm???? Haynes says 1mm. some people on here even use less! I can try backing it off a bit more but 12mm seems like that would create a huge travel at the pedal end! The bushings are the original plastic but there is no sign of binding. The pedal returns all the way when you take your foot off. Also the 19mm was from here at Pelican Parts. I assumed they were all rebuilds. Do they make brand new ones?

srb7f:

That is interesting! Maybe I don't have a problem at all??? I assumed that once I set the venting clearance on the rear pads that I would continue to have that clearance after appling the brakes!!! What I mean by "dragging" is if you could imagine putting the wheel back on while it's in the air and giving it a spin, it would stop immedately when you took your hand off. You could spin it yes but it certainly wouldn't continue to spin. Maybe brakes work diffently on 914s than they do on my other cars! Is there anyplace I might have read about this behavior before wasting all this time???

[darisb]

I had the same problem with the new master cylinder that I put into my VW Bug. Had the same trouble with a Bug I had years ago. The guy at the shop told me it was air in the master cylinder. I know with American cars you had to bench bleed the master cylinder before you installed it. Kind of tough to do when you don't have a reservoir on the master cylinder. This time I bought a power bleeder from Motive Products. Best $50 I ever spent. I believe our host sells them. I pumped up the bleeder had the system bled in no time. Worked great. Don't think it will help your dragging problem, but pretty sure it will fix your soft pedal.

Regards,
Daris

[lmcchesney]

Sean,
Check out this post regarding proportioning valve and "T's".
The proportioning valve revisited ...
Have you solved the spongy problem?
L. McC

[Lyle]

Sean,
This is Lyle again. the 12 mm I referred to in my first response is the distance the brake pedal pad can move without actually applying force to the MC pistons. There is roughly a five to one leverage built into the design of the pedal, so that translates to roughly 2mm of clearance if you measure it at the piston/rod. Lyle

[SGomes]

I assume people are refering to me when they say "Sean"... (my name is Shannon )

Daris:

I couldn't agree more! I purchased the same pressure bleeder...great investment!

L. McC:

Yeah read that post. Wow VERY thorough! I always wondered what was inside that thing. I haven't completely solved the spongy problem but it is WWWAYYYYYY better than it was with the old MC. There was definitely a problem with the old one. I suspect I still have some air in the system somewhere.. probably the Pvalve. It is drivable right now I guess.

Lyle:

Thanks for the clarification. In that case yeah I've got something in the 12mm ball park.

What is really intriguing me right now is the post from srb7f... Perhaps I shouldn't be concerned with the dragging at all! The car isn't ready to go back on the road yet so I can't test the theory. Has anyone else had this experience? Brand new pads on brand new rotors will drag until you drive it a bit????

[tat2dphreak]

they may drag a bit until you drive the car a ways... but they shouldn't drag much... I wouldn't think so anyway

[srb7f]

Shannon-
If the wheels stop immediately when you stop turning them by hand, there is definitely air somewhere which is preventing your pistons from retracting completely.
By dragging slightly (normal amount) you should be able to hear the pads drag against the metal. This will cause significantly more friction than no pads at all, but even so, they should be able to spin for at least a little while from the momentum of the wheel after you let go.
You may also find that your pedal will feel better if you just forget the rear venting clearance and adjust the pads (closer) to the point that they sound and feel the same as on the fronts with a similar amount of drag. You will however have to get the air out first. Goodluck.