MK2 brake puzzlement

[Deleted member 8987]

I've worked on vehicles hobby and professionally for 45
years. Worked on more British cars than I can remember. But
I've come across something I cannot seem to figure out.

My MK2 was poorly maintained (gas station) for 30 years.
Brakes was one of the issues.
So, I had the master re-sleeved and rebuilt. All new
flexible hoses. Cleaned out the reservoir, new float
assembly, new line from reservoir to master, new line from
master to servo booster, couple of other steel pipes, new
calipers front (had the wrong sized pistons), rebuilt rear,
new pads, then the PBR (Australian) power booster
rebuilt....and continuously got air in the servo cylinder.
Played with it for about a year, you'd know, as the pedal
got a tad ''mushy'' at the top. No big deal, crack the bleeder
on the servo, wait until the air was gone, good to go for
another month.
Rebuilder in Melbourne couldn't figure it out, so sent the
cylinder and pushrod back, he went through it again
(different cylinder came back), and it's been fine. High
firm pedal, good boost, no issues.
Saturday morning, drove it to St. Arbucks, no problem,
parked it till this morning.
On the way to St. Arbucks, geez, pedal a tad low, a little
soft, no warning light. On the way back, complete loss of
brakes. As in, pedal to the floor. Pumped a couple of times,
nothing, crept home (couple of blocks) on e-brake. Reservoir
full, right where it's been all this time. Cracked the servo
bleeder...air. Let it evacuate, pumped a couple of times,
re-bled, no more air, works good, drove it, back to her old
self.
I have never seen air work it's way to a low point like
that. If the master is letting air in (my experience is that
is quite common when they start to go, being almost
vertical), it won't migrate downhill to the slave. Since
it's not using any fluid, and nothing is damp, I am just
throwing this out there.....I am now adding a weekly servo
bleed to maintenance. Odd.
Thanks
Dave

 

[DrEntropy]

As you say, air in the system would find its way UP, not migrate down. Gotta be "inhaling" somehow, someplace nearby on pedal/pressure release. Fittings first, IMO.

This is one of those things to drive ya crazy.

 

[Deleted member 8987]

Yeah, crazy ain't adequate at describing it.
Checked fittings for tightness, all okay. Checked fittings for dampness...none. In fact, everywhere I look it's powder-dry.

The master is oh, 14" above the booster, and 24" aft of the booster. The next fitting in line is the 5-way fitting on the inner wing, which is six inches above the booster cylinder.

Monitoring.

 

[DrEntropy]

Mebbe some "pipe dope" on/around the threaded fittings, one at a time, until the intrusion point is found?

Starting with the lowest one first, methinks. :wink:

 

[Silverghost]

Mebbe some "pipe dope" on/around the threaded fittings, one at a time, until the intrusion point is found?

Starting with the lowest one first, methinks. :wink:

I had a similar thought but wasn't sure that was done on cars/brake systems. The other thought I had (used my quota for the month!

) was is air getting in through the sensor/fitting for the warning light? Isn't that mounted up high in the system? You mentioned it didn't work....?? I may very well be wrong....

 

[Deleted member 8987]

The sensor is a wine bottle cork on a rod fit to the cap of the unpressurized reservoir.
Having done auto stuff for 45 years or so, pipe dope on flare (or bubble) fittings always seemed to be a waste of time, since the sealing part is the actual flare/bubble.

I am leaning towards the pushrod seal, when the car is off, relaxing and letting air into the bore, but that's just my one allowed thought for the day.

Bourbon time.

 

[DrEntropy]

Yeah, I ~know~ the flare is th' seal, TOC. But <span style="font-style: italic">you</span> know (and have likely seen a few) the flare can crack. Usually resulting in immediate leaking... but not always.

The pipe dope is a less labour-intense method than full disassembly/inspection, IMO. And it would rule out one possibility.

 

[MikeP]

If you do that, and I'm not recommending either way since I've never tried it, you'll need to have it both on the threads and on the pipe going through the fitting since that's not an airtight fit either.

 

[Deleted member 8987]

I know about cracking.....something to think about, and yes, I knew inside and out of the nut.

 

[DrEntropy]

I know about cracking.....something to think about, and yes, I knew inside and out of the nut.

I assumed you'd know to "seal" the pipe-to-gland along with the threads. :wink:

 

[Deleted member 8987]

Yup. The odd thing is (and the rebuilder and I discussed it) was no leaks. No wet fittings. The chances of a fitting sucking air and NOT being at least damp are small....very small.

 

[LarryK]

Have you thought about the brake booster? You are not loosing fluid and can find no leaks. Just a thought of the possibility of loosing boost which will let peddle drop and soften up. Vacuum lines? Booster diaphram? Like I said just a thought since it is located in fender and long lines to the system.

 

[Deleted member 8987]

Well, ket me give you the "history". I did all the rest, master sleeved and rebuilt, cleaned the reservoir, new lines reservoir to master, master to booster, new flex lines....then new front calipers, rebuilt rears.....new pads....and the booster died.
Bypassed it, pulled it, wondered whatintheworld it was. Had to google it. PBR unit, made in Australia. When a Brit Mobile is in Singapore, where do they get parts? Aus.
So, packed it up and shipped it off to Melbourne (nobody stateside knew what it was or had parts), got it back.....and it worked good...except I would occasionally get a little mushy on the pedal, cracked the bleeder on the booster, vented the air, and all was well until next time. Then, it started "pumping". Roll up to a stop, let off the pedal a tad as you are just stopping, pedal comes all the way up, and brakes lock. Let go, pop, brakes release, worked fine.
Got a new reservoir (removed in Singapore), and check valve, and hoses, mounted and checked operation, plumbed correctly, just to eliminate any possible lack of vacuum as being a cause of the "pumping".
Finally yanked cylinder off, sent it back (rebuilder who specializes in PBR's never heard of such a thing...and kept telling me I hadn't "bled" the system right, which I had...several times). Got a different cylinder back, re-did the bubble flares on the lines to make sure, installed and bled, worked good with no air showing up for, oh, 3 or 4 months anyway (checked periodically) until this happened. Halfway down the hill, pedal got a tad soft, turned around bottom of hill, NO brakes at signal. Air in booster cylinder. Bled it, fine, good brakes. I bled it several times since, always air, but no more loss of brakes. Now (today) getting a bit of the "pumping" again.
I think I need to find the correct unit somewhere and get it rebuilt.

 

[Silverghost]

Just "thinking out loud" here...could there be some rubber component(s) which either due to incompatible materials or incompatible fluids, etc. break down over time in such a way that little by little air gets in but no fluid leaks out....almost as if a one way leak "valve" is created?? ie an o-ring that rolls in one direction creating the leak but when it moves back it seals up again, thus no fluid leak?? Like I said..."thinking out loud"....

 

[Deleted member 8987]

Thought of that. The folks who rebuilt this PBR unit specialize...and they've never seen anything like it. Now, the second cylinder is doing odd stuff.
Driving it today, I noticed rolling to a stop with light pressure on the pedal, it racked right up until it was tight...let go, pop, all is well.

 

[Deleted member 8987]

I have thought...probed..thought some more. Since the booster is a low point, by about 4" to 5" below the junction block, any air has to originate at the booster or the fittings.
I had replaced the pressure line into the booster from the master, so, tonight I made a new output line from the booster to the junction block. I used good, non-flared-out fittings from the caliper jumpers, and my bubble-flare tool. Installed and bled it down, drive it tomorrow and see what I get out of the booster bleeder.