this ever happen to you????
so greg and i bleed our brakes on both cars......
next day i go for a short run, pledal pressure gets harder and harder.....
soon the rear calipers ar full on without me touching the pedals
so i pulll of the road...pull the wheel let a little pressure out of the system....and off i go....
next day same thing......
gregs 914 expeiences the same problem....
my system is modifide a bit, 19mm, sc calipers up front, rear bmw calipers, and a porportioning valve screwed in one turn...
gregs- 19mm, sc fronts and the rest untouched....
any ideas???????

Sounds normal. Turn the proportioning valve in another turn or two. With those BMW rears you're over-braking the back having the valve nearly wide open. Ideally you would want a bit less braking on the rear so that the fronts lockup before the rear (if they lock).

Sounds to me like the master cylinder is not comming back all the way and there is some residual pressure being maintained in the system. When the cylinder comes all of the way back up it should vent to the reservoir and release all of the pressure. Possibly something in there came loose during bleeding.
HTH
Harvey

i have been turning it a 1/4 turn at a time....nextime....i will turn it in one turn....
but does that explain both 914s locking up???

perhaps one of the seals slipped? eh?

any new revelations on this

This is where I'd recomend a T.

With A-Calipers up front and BMW calipers in the rear you have a serious mis-match. What are you guys using for a hand-brake?

If a hand-brake isn't an issue, I'd recommend rear M-Calipers with (vented) or without (solid 914-6 rotor) the spacers.

Sounds like pressure is building up in the P-valve and eventually causing the rears to lock. This may be from your adjusting it... maybe not. P-Valves usually never fail.

The only other thing I can think of would be the dual circuit 19's you guys rebuilt. You sure they went back together right?

Eric nailed it!!

I think it is something in the MC.. and I agree about the prop valve. Lose it.


B

You have adequate pedal pushrod clearance? The Cap'n, who NEVER rebuilds master cylinders. EVER. If they're bad, there's a reason, and the typical rebuild NEVER addresses those reasons.

It sounds to me like the rear flex lines are acting as a one way valve. The MC develops enough pressure to force fluid past a line restriction, but that same restriction does not allow the pressure to bleed back. Either that, or as the Cap'n says, the pushrod does not have enough clearance to allow the guts of the MC to return to normal position and allow the pressure to be released. Perhaps the guts of the MC are stuck though.

Exactly how much of a rebuild on the whole system have each of you done recently?

I believe that Harvey answered it first. It sounds like the brake push rod is screwed in too far not letting the brake master cylinder to fully "release" the brakes. I had a similar thing happen to me once about 13 years ago. The pads would rub and generate heat until the fluid and pads swelled enough to cause the wheels/brakes to lock.

Backing out the push rod 1/2 turn was all it took.

It is not your proportioning valve UNLESS the rears lock before the fronts in a hard braking excercise (threshhold braking done in a controlled environment).

we have not rebuilt either the mc.....( on my car)the p-valve is in a turn and a half in......
eric i think i am going ot go back to stock 914 calipers with spacers for vents....
i will pm you on that.....
perhaps it is my lines ....
perhaps it is the mc
can one damage a mc during bleeding?????because this is the only thing that changed.....the mc was soft so we bled the whole system...and after wards had the problem....
planning my day in the shop tommorrow......

Dave has a good point but maybe not both cars

John also has a good point about the rear pads rubbing. We saw this once with a teener over yonder... (can't remember... Amsterdam?) Pads rubbed and cause the whole thingy to swell and lock up.

I was assuming the rebuilt M/C because I sent two kits that-a-way.

Thought... maybe something "is" happening with the P-Valve:

a ) It's only the rears
b ) You didn't mess with the M/C
c ) It seems to act like Dave's rubber line thing.

Maybe the P-valve is retaining the pressure on the lines like the blistering rubber line would. I had the rubber line failure happen to me once and although both front and rear lines were bad, only the front locked up. I would imagine there wasn't enough pressure to engage the P-valve. I'm wondering if it has something to do with you adjusting the p-valve...

the lines are relitively new...like two years...but i can replace them....but how would i check them?

I doubt it's the lines.

Yes, you CAN damage the master cylinder during bleeding if you let it bottom out. The Cap'n

perhaps i bottomed it out but how can i be sure?
did we blow up two mc ...makes for an expensive evening in the shop.....
well my education is continuing

this mc had less than 10000 miles on it

Let me get this straight.

1. You did NOT change Master Cylinders in either car.
2. You did NOT adjust the brake pushrod in either car.
3. Both cars exhibited the same problem AFTER only bleeding the brakes.
4. The pistons in all calipers retract easily (don't cause the brakes to drag).

Here are a few questions:

1. In your buddies car (with stock rear calipers). Did you set the venting clearance on the pads?

2. Do you have new brake pads in your non-stock rear calipers?

3. How much clearance is there between your rear pads and the rotor (on your non-stock rear calipers).

4. What kind and type of brake fluid did you use when you blead your and your buddies brakes? Was it silicone (DOT 5) or was it non-silicone (DOT 4 or less)?

5. What brake fluid was in each of the cars? Was it silicone? Was there a mismatch in brake fluid causing some coagulation of the fluid in the system?



With the above thought process, it seems that maybe there might be some brake fluid incompatablity between what you used and what was in the car(s).

You may try flushing the brake fluid (2 quarts minimum) before changing all kinds of other variables. (This is assuming that the brakes did function prior to your brake bleeding)


just my $0.02

big thanks for your two cents....!!!!!!
yes both cars where perfectly operable before bleed other than a soft pedal on both cars.....greg ( my brother pete-greggers) car had a soft pedal
and mine seemed soft as if the mc was leaking back....but after bleed was solid...
yes we did use two different fluids....i think i had some cheap stuff in mine, and added pentosin dot 4 after wards...(if memory serves )
those are the only two varibles
- brake fluid
and -bleeding of brakes
there was no other changes!
we did not reset rears calipers on gregs car
and i am running a front caliper with no adjustment( i think)
the problem is - after driving for 15-20 min the brake pedal gets harder and harder- then the rear calipers lock down...with high pressure in the lines,

Remove one rear rubber line and see if you can blow through it. Since the system was working correctly before, there should be no restriction.

Can you return the prop valve to its original configuration? Does that help?

Was there any crap in the reservoir before beginning to bleed that may be restricting the MC operation or plugging the lines? I ALWAYS empty the reservoir and refill before bleeding.

If you have crappy old fluid in the system, then rust can form anywhere and cause problems. If the bore of the MC is rusted, then pushing the piston past the point of the rust (where you normally don't go) can cause damage to the seals, or possibly hang up a section of the piston.

so how does one drain all this fluid? i am hoping this solution is the ticket....

I would NOT recomment draining all the fluid.

When I flush brakes each spring, I start out bleeding the passenger side rear.

I bleed the brakes until the reservoir to the rear is almost empty but not completely.

I then move to the passenger side front and drain the front portion of the reservoir almost empty.

I then fill the reservoir with fresh DOT4 fluid and move back to the passenger side rear. I fill and bleed this corner until I am sure that I have fresh fluid (a pint or so should be more than enough). This flushes all the old fluid out of the master cylinder back through the tunnel and to the proportioning valve and the passenger side lines and calipers.

I then move to the drivers side rear caliper and bleed that caliper (again about a pint should be good to flush it out. Your rear brake lines, proportioning valve, and calipers should now have fresh fluid in them.

Then move to the passenger side front caliper and bleed about a pint through there.

Finish up by bleeding about a pint through the drivers side front caliper.

You should now have completely flushed your brake lines (and used about 2 quarts of brake fluid).

That's how I do it.

While you are at it, please do disconnect and blow air through those rear rubber brake hoses and make sure they are not blocked. It would be a pain to do all this work and still have a problem.

Good luck to you.

Pete, you still coming down Sunday? If you don't get this figured out by then, just drive on down and don't use the brakes. We'll take a look at those as well.

dan i plan to be there.....!!!!!!!!
i will get this sorted tonight hopefully...

well........
after much chin wagging and wondrering we decided to drain some brake fluid.....
and guesse what was found....two fluids in th e the line of two different colors and weights....they seperated in the container....
i figure it was a "gunk dot 3" and a mix of "pentosin dot 4"
those where only two fluids in th e shop.....
strange little coagulations inside the fluidn as well.....
which would explain the mc valve acting up...(hopefully)
we should get this completely sorted tonight..with a spare mc on hand jsut in case
the morale of the story...don't have cheap ass brake fluid in the shop!!!!
and if you do ...don' t mix it!!!!!!!!!!

I've never seen fluids like that. Not unless one was a silicone fluid. There was a thread on those fluids not mixing recently, pictures included.

I think DOT 3 & 4 mix just fine. I believe DOT 5 doesn't mix with the other two.

If the brake are seizing after driving you are dealing with a heat induced problem.


There's no venting clearance to set on the rear BMW calipers is there? I don't think so. So you've got no e-brake, right?

I don't buy the proportioning valve theory. Assume the brakes worked before. How would proportioning valve cause the calipers to stick closed? I can't find the schematic of the prop valve but this seems like an unlikely failure mode.

I think that the MC operating rod is set incorrectly. Prior to you bleeding the brakes you said you had a spongy pedal. With a lot of air in the system you could probably drag one or more brake pads, due to a incorrectly set MC operatiing rod, and get away with it. With air in the system, it would manifest by feeling like progressively worse braking as the H2O in the brake fluid, due to air and old fluid began to boil. However, there was so much air and H2O in the system that it never generated enough pressure to clamp the calipers on to the extent you noticed. You could compensate by pumping the brakes as well. With bleeding the brakes you've taken enough of the compliance out of the system and now with dragging brake pads you actually expand the system, through heating, enough to clamp on the brakes.

I bet all four calipers are clamping down.
Check the free play in the brake pedal. It probably doesn't have enough. I wouldn't blame the fluid, other than to say it should have been changed at some point in time..

If this isn't it I'll deny everything.

Later,
Kevin