Brake problem

[oscar]

The obvious thing would be to now test again by putting the pedal in a bit so the piston/s move into the worn areas, then exclude the lines by putting the bungs in and try the same test.  If the pedal sinks, it'll confirm that fluid must be going past the seals.

I did this test this morning, result was not what I wanted.  I tried a few positons of the pedal to halfway in and allowed fluid to bleed from both circuits then tightened bungs again.  Statred the engine and the pedal was firm from all those positions, I could not get the pedal to sink.

New MC arrived not long ago, something tells me not to expect too much.

[TJ 450]

I suppose, if the master cylinder change has no effect, then I would disconnect the rear brake metal lines at the tee fitting and plug that with one of your bungs. That would also be the perfect chance to fabricate the new lines. Ideally, it would be good to plug the front ones as well, then you could isolate the problem to be in the steel lines themselves. BTW, do you have that brass box contraption hanging off the rear brake line, next to the MC, or is that 6.9 only?

Tim

[oscar]

BTW, do you have that brass box contraption hanging off the rear brake line, next to the MC, or is that 6.9 only?

Tim

No I don't, and I meant to ask you about yours in your thread after seeing it hiding behind your reservoir.  Thing is, the brown 280 had one with a bung in one end.  It had three outlets, I thought someone had turned a brass block into a t-piece.  I'll find it and show a pic tonight.  I'm not sure it's exactly the same as yours but what's it for?

[koan]

do you have that brass box contraption hanging off the rear brake line, next to the MC, or is that 6.9 only?

What's this?

I think I remember seeing a brass object in one of your pictures at some stage.

I don't have one, my three lines just come down from the M/C and head off.

koan

[oscar]

Nup, mine's tiny, it's different, but maybe they're supposed to do the same thing, whatever that may be.  One thing I know is that on mine, the nuts were shorter and smaller spanner, possibly imperial measurement.  I'm sure it's not OEM.

TJ's is the second photo below taken from Brakes and a few small issues

[koan]

I've got a couple of the exact same thing oscar,  they are T-pieces or 3-way junctions, we used them redoing the brakes on Julians's mini, they are imperial thread.

I think TJ's is probably a bigger version of the same thing.

A repair for damaged plumbing?

koan

[oscar]

Guess what, new MC made no difference.   
I'm surprised I haven't thrown any tools yet, smashed some windows,  kicked the dog or headbutted the wife.

I'm going to go with TJ's suggestion now.  Rear callipers and hoses are the only thing I haven't changed which in hindsight, I should've done the rear hoses straight away but as you've probably read, I have that little issue of rounded and seized fittings on the steel lines.  So I'm off to grab the red's rear lines from that T and put them on povo, but will block the T and see if it makes a difference first.  I'll have to settle for the new cheap ebay flexible hoses for the moment as the MB ones I bought a little while ago have two female connections, not M/F.

[TJ 450]

Guess what, new MC made no difference.  
I'm surprised I haven't thrown any tools yet, smashed some windows,  kicked the dog or headbutted the wife.

That would be infuriating.

That brass box appears to be a T piece with a bung or bleed valve in the top... I have always thought that it went with that type of master cylinder. I also thought that it was there to bleed the system, perhaps it is a repair piece.

My 6.9's pedal still sinks almost to the floor and I'm pretty confident that it is the rear hoses. I'm having a break from the 6.9 now, but soon I will need to locate some brake lines (I already have some hoses) for the same reason.

The F/F hoses are for the anti-squat rear.

Tim

[oscar]

Still no good.  I changed that steel line to the left rear, had to hacksaw through the old line and the metal spring thing betwen the line nut and the female hose connection to get the old one out.  Was about 02:30 when I got a replacement in.  So I put new rear hoses on finally, but no improvement.  Now I've just put spare bendix callipers on which used to lock the red's rears up too easily.  Still no improvement.

Done a full inspection as best I could of all steel lines and there's no sign of corrosion.  I let the car run for a while, marked the fluid level and kept operating the brake repeatedly till it kept going to the floor.  Did that for a couple of minutes and there was no fluid loss.  I think it's back to the callipers and maybe rebuild a set again.  I had a crazy thought though that maybe the the reservoir cap is supposed to be like a one way valve so I changed it, nothing.

The brake pedal feel whilst engine running is still that it gets firmish halfway down then sinks with sustained pressure.  

The F/F hoses are for the anti-squat rear.

Thanks for that.  Didn't realise that was the determining factor.

[craigb]

Sorry for stating the obvious but there has to be an answer here and I can't picture how the caliper can be it. Even if it was air, why would it hold off and suddenly compress. It seems like under pressure fluid is somehow bypassing in the master but that is new. There isn't any other proportioning valve or anything where under pressure fluid can move to a space without leaking out anywhere? If we picture we have this non compressible fluid taking up all the spaces, for it to hold pressure and then go down later, that volume must move somewhere. Just thinking out loud here in case it triggers any other thoughts. I will keep trying to picture it and let you know if I have any bright sparks.

[oscar]

I hear you Craig and I agree.  I replaced all pads now, two new OEM ones up front and two nearly new OEM pads on the rear.  Pad movement or piston movement seems negligible.  I can't see pistons move at all compared to what the original callipers at the beginning of all this were doing.  So it doesn't seem like it is callipers.

Two other things crossed my mind, fluid and brake booster.  The fluid's Castrol Super Dot 4 but I might try a cheaper Dot 3 whilst I troubleshoot.  I've tried to look at viscosities on MSDS forms but some are inconclusive. I really don't think it'll make a difference going to DOT3 but I'll try it.   Also, I've been playing with a spare booster.  Mine does hold a vacuum and doesn't seem to have any leaks, but on the spare I was looking at the pedal attachment and the actuator that fits onto the master cylinder.  These aren't physically connected or don't seem to be on the same shaft.  But they don't seem to separate either.  I doubt the booster's to blame even though it's functioning is what makes the pedal sink.  The pedal without engine running is stiffer at the moment.

[beagle2022]

I know it isn't kosher to resurrect an ancient post, but the quality of the discussion here was fantastic.  My frustration is that I am facing the same problem: spongy brakes.  The thread just seems to stop when they were getting close to an answer.  Has there been an epiphany on the 116 spongy brake issue??? Am I doomed to replace everything (like these guys did) and not resolve the problem??

In my case after nearly four years of perfectly behaved brakes, the pedal is becoming spongy.  No loss of fluid and no leaks.  Relatively new MC, booster, lines etc.

Any clues on where to start?