funny stuff, after brake bleed, rears locked up.......twice Options

[pete-stevers]

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???????

[Brando]

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).

[HarveyH]

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

[pete-stevers]

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???

[pete-stevers]

perhaps one of the seals slipped? eh?

[pete-stevers]

any new revelations on this

[Eric_Shea]

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? (IMG:style_emoticons/default/confused24.gif)

[Brad Roberts]

Eric nailed it!!

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


B

[Cap'n Krusty]

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.

[davep]

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?

[John]

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).

[pete-stevers]

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......

[Eric_Shea]

Dave has a good point but maybe not both cars (IMG:style_emoticons/default/confused24.gif)

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... (IMG:style_emoticons/default/idea.gif)

[pete-stevers]

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

[Eric_Shea]

I doubt it's the lines.

[Cap'n Krusty]

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

[pete-stevers]

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 (IMG:style_emoticons/default/sad.gif)

[pete-stevers]

this mc had less than 10000 miles on it

[John]

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

[pete-stevers]

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,