spontaneous brake failure

herr_sparky · Post by **herr_sparky** » Fri Mar 06, 2009 3:37 pm

had a strange thing happen yesterday: i finished bleeding days ago and have taken the car out 3 times, making several hard stops, and everything was working fine. better than ever, actually, since the only components that aren't new are the drums and the rotors. i needed to turn the car around to face the other way in the driveway, so i started it up and when i stepped on the pedal to take it out of PARK, it felt a little strange, so i stepped harder and suddenly it sank all the way to the floor...good thing i wasnt driving when this happened...

one of the elbows going into the MC is leaking around where it snaps into the rubber bung, its the rearmost one. the other one is fine. this is the only leakage i can find, and its just barely enough to have left a tiny drip on the floor.

what the hell? i've started over with bleeding, but theres been no noticeable improvement yet. there's ZERO pedal pressure and everythings tight & dry...ideas?
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Post by **david58** » Fri Mar 06, 2009 4:06 pm

Was the reservoir low? If it was look for a leak in the lines under the carpet. If full might be the master cylinder bypassing the fluid internally.

wildthings · Post by **wildthings** » Fri Mar 06, 2009 5:19 pm

Could be that the cups in your master are shot. Time for a new master or a rebuild kit, whatever you can find. There was a NOS master sitting on a shelf at Parts Obsolete near McMinnville several years ago. You might give them a call.

herr_sparky · Post by **herr_sparky** » Fri Mar 06, 2009 7:06 pm

i know, it sounds like it could be either/both of those things, but the res was full and the MC was/is NOS (i disassembled, inspected, and cleaned it before installing it. the seals were fine and the bore was like glass).

i've removed the rear wheel cylinders and they were both a little sticky...in fact one of them was very dry inside. i'm starting to think that maybe one or both of them was stuck and finally broke free that morning...would that make sense?

and the leaky elbow... wouldn't fluid that was being pumped by the MC ooze out the elbow/rubber grommet if there was nowhere else for it to go in that circuit (because of a frozen piston)?

at any rate, i've finished cleaning out and prepping both wheel cylinders and tomorrow i'm going to bleed again. this time i'm going to lock to shoes against the drums with the adjusting stars like some of you have recommended.

Post by **david58** » Sat Mar 07, 2009 6:24 am

herr_sparky wrote:i know, it sounds like it could be either/both of those things, but the res was full and the MC was/is NOS (i disassembled, inspected, and cleaned it before installing it. the seals were fine and the bore was like glass).

i've removed the rear wheel cylinders and they were both a little sticky...in fact one of them was very dry inside. i'm starting to think that maybe one or both of them was stuck and finally broke free that morning...would that make sense?

and the leaky elbow... wouldn't fluid that was being pumped by the MC ooze out the elbow/rubber grommet if there was nowhere else for it to go in that circuit (because of a frozen piston)?

at any rate, i've finished cleaning out and prepping both wheel cylinders and tomorrow i'm going to bleed again. this time i'm going to lock to shoes against the drums with the adjusting stars like some of you have recommended.

Yes that is would cause pedal loss. It isn't common but I have seen that happen recently. I gravity bleed the brakes on my 74 after replacing the hard line inside the car. It had a full pedal. Even though, I didn’t get good fluid flow out of the rear brakes. I had left the rear bleeders open for a week, I forgot they were open and pumped the pedal real hard and the pedal went to the floor. That is when I realized the bleeders were still open. These wheel cylinders are old, maybe even original. If a wheel cylinder is stuck fluid can bleed out, you can get a good pedal. If there is no movement of the cups then the wheel will still spin. When the cup frees up you will have pedal loss. This could well be a gremlin that haunts us buying NOS parts. I have seen brand new wheel cylinders stuck sometimes just one of the cups. This causes a spongy pedal sometimes and brake pulls that are hard to diagnose.

raygreenwood · Post by **raygreenwood** » Sat Mar 07, 2009 11:00 am

If there is no air in the syetm...even though a cylinder is stuck...yes...you will lose pedal when the cylinder finally moves....but only once. The volume lost as the pistons move will be made up on teh next stroke.....IF...your compensation ports are working in the master cylinder. It pulls spare fluid from the resovoir on the next upstroke....and all will be well.

Fluid leaking from a bung means that air is leaking as well. This is a "0" pressure area. If fluid can get out...air can get in.

Also....if its an NOS cylinder...and its over 2-5 years sitting on a shelf....the seals are indeed shot. I have not found one yet that worked well for any kind of milage...if at all. The bungs are the same age and in teh same contracted, shrunken up and less elastic condition. Thats whay they can leak.

Do this: Pump up the pedal very quickly. Hold it with hard foot pressure. If it stays solid......now....ever so slightly REDUCE foot pressure by about half but do not take your foot off the pedal. Then give it hard pressure again. If it now sinks to the floor quickly or slowly....your seals are indeed hardened and have taken a "set"...and collapse when pressure is released.

But...if you have a clean master cylinder bore with pistons and flaps in good shape....locate a German type 3 cylinder or kit of tehesame make (Teves or FAG). Also a rabbit dual circuit with 19mm bore of correct makeas well....is a great donor. Pull out the seals...carefully and discard the rest. Drop them in...and your cylinder will be new. Works perfectly.

On that note: www.rockauto.com lists 411/412 master cylinders new (wagner) and rebuilt (A-1 cardone) for $28 each
They also list reman type 3 cyls for the same price. They usually have pretty good parts.

And....Olympus....a Beck Arnley dealer in my area with excellent parts quality lists brand new 411/412 cylinders (Centric brand..not bad..about like Vargas but a little better) for $218

http://oip.webwareauto.com/Catalog/CatalogResults.aspx
But:.....heh heh...

they also list an ATE repair kit for 73 type 3...for $25 http://oip.webwareauto.com/Catalog/CatalogResults.aspx

Even though ther are very slight differences in ATE to FAG...they are generally in pistons. I have used with good success a seal only kit form an ATE in an FAG cylinder. The only thing that may get you into trouble was on a very few pistons that the height of the seal on the outer shaft seal portion was a few thosuands difference.

If you have an ATE cylinder...buy this kit and you are done. If you have an FAG cylinder...I think $25 is worth the risk. Ray

herr_sparky · Post by **herr_sparky** » Sat Mar 07, 2009 7:56 pm

damn! i knew you'd say that...its the only logical explanation, really.

i knew there was something afoot when i couldnt get the system to "pump up" at all, just in denial because i didnt want to take the MC out again. i thought the seals seemed pliable enough when i inspected them, but i didnt really have much of a frame of reference, either. they werent cracked or rock-hard, so i figured they were fine.

just for laughs, i'll share some of my observations about the wheel cylinders:

inside the one that had very poor fluid flow (like david58 described) there was an apple seed-sized black spot of corrosion/discoloration on the bottom of the bore, right about where the edge of one of the seal-expanders rests. the rest of the surface finish was still mirror-like, and a few passes with 2000 grit sandpaper got rid of all of it except for a tiny distortion in its reflection. in fact, the seal expander itself had a corresponding mark on its edge like a dent or a wrinkle. i figured that a very small amount of moisture was trapped there and caused enough of a corrosion bump that when the seal/expander contacted it during the initial filling/bleeding, it became crooked in the bore and was prevented from moving through its full stroke. after being heated up from my test drives around the neighborhood, it finally broke free and caused the pedal to sink... BUT: as i understand it, i should have been able to recover at least some pedal pressure after that initial sinking... after several stabs at it there's none.

there is one other peculiar detail to this whole saga: on the same side as the cylinder with the black spot, when i removed it the hard line leaked no fluid. zero. i even hooked my vacuum pump up to it (with the rubber cone fitting into the hole to prevent any leaks) and drew upwards of -50 kpa and got nothing but air. for a minute or two. wha the? maybe i'm just suffering from dementia brought on by too much exposure to DOT 3...

any other thoughts from anyone would be swell, and thanks for the replies, i always appreciate an extra brain or two...

herr_sparky · Post by **herr_sparky** » Tue Mar 10, 2009 1:37 pm

ordered rebuild kit from rockauto.com today, only $18.

just to be absolutely sure, i have a couple questions:

from left to right, flap valve, seal, spring seat& spring. on my other pistons the spring seat & spring fit snugly on the piston, on this one it is loose. i dont see how this would affect anything since its all held together with spring pressure once assembled, but maybe i'm missing something?

also, i want to be sure the rubber bungs are installed correctly. on the left is how it was, but Ray has said that sometimes they are in upside down. it would fit pretty poorly if i flipped it around so the lettering was up, but just to be sure...

raygreenwood · Post by **raygreenwood** » Wed Mar 11, 2009 5:31 am

Yes, every once in a while I have had the problem that if a spring is not seating properly it could be causing a flap valve not to work. You can generally clench that friction keeper part a little with pliers and get it to grip the pin on the piston better.
I would polish the flap valve on 2000 grit...circular motion... and try it again. Look for any corrosion specs around any of the ports under the flap.

the Bung picture on the left is correct. Make sure the elbow does not bottom against teh cylinder....or cut vent notches in the edge. Ray

herr_sparky · Post by **herr_sparky** » Sun Mar 15, 2009 12:09 pm

the seal kit arrived the other day and what a difference! side-by-side its obvious that the old, NOS seals are collapsed:

i also discovered a very clogged rear brake hose, it was almost completely blocked. now all 4 hoses are new and i checked every section of hard line with the vacuum pump for good flow...

i almost made the mistake of using standard fuel hose for the blue stuff, but checked the STF archives at the last second and found out its brake fluid-specific. i would have freaked out when it started leaking in a day or two...damn that stuff is expensive when you need 26" of it...

my question is: where can i find replacement rubber "bungs" for the MC elbows? everything that i have seen so far online is the wrong shape. what about just sealing the area with silicone or something?

raygreenwood · Post by **raygreenwood** » Sun Mar 15, 2009 4:28 pm

By the way...what were the part #'s or words inscribed on the top of that seal on the right side of the picture? It looks like a "schaefer" part almost. And...if I might inquire...what was the brand of the master cylinder? Ray

herr_sparky · Post by **herr_sparky** » Sun Mar 15, 2009 6:15 pm

its ATE... the numbers are not entirely legible, but they look like:

3.3304-11004 46407 5-4

raygreenwood · Post by **raygreenwood** » Tue Mar 17, 2009 12:51 pm

Those look pretty normal for ATE...the squared edge. Also, the height will usually not be the issue..though it may have lost some height. Usually the height different will mean a leakage issue....because if the seal is not compressed enough for its skirt to properly seal...it leaks around the shaft in the center.
That "skirted" type of seal is called a shaft seal.

Its impossible to tell collapsed or old seals from appearance usually. Its not so much that the shrink in size...they shrink a bit internally in porosity and flexibility. Because of this, tehy do not "inflate" as well from pressure on the cylinder walls.

If you still have your cylinder out.....and you have a bore gauge or a snap gauge with a micrometer....I would be curious to know teh bore diameter.
Its another common problem that crappy rebuilders will over bore...or over hone type 4 cylinders to get rid of rust pits that are common at the outside of the bore. This makes the bores too large in diameter to proerly seal....especially on the squared skirt ATE cups. Ray

herr_sparky · Post by **herr_sparky** » Tue Mar 17, 2009 1:54 pm

its already assembled and in the car (mostly), just waiting for new plugs for the elbows, now. it's supposed to be new, not rebuilt, at least thats what the seller claimed...

herr_sparky · Post by **herr_sparky** » Sun Mar 22, 2009 11:55 am

update:

now that all the fresh parts are installed the brakes bled up just fine. 3" pedal travel and very solid feel...nice to have an e-brake again, too.

so the list of gremlins looks like this:

bad MC cup seals (renewed with T3 seal kit)
bad MC elbow grommets (replaced with new T3 grommets from busdepot.com https://www.busdepot.com/details.jsp?pa ... =311611817)
stuck wheel piston (cleaned, reused)
clogged rear brake hose (replaced all 4)
dirty/clogged reservoir (cleaned, reused, replaced blue hoses)

thanks for the help...