Soft brake pedal - system bled, no leaking fluid - MC?

[Salinity]

Hey y’all - Did a search, but most info seems to indicate a failed MC would result in a visible leak (is this correct?). Trying to Susa out if that’s my issue - Here’s my situation:

Soft pedal - will stop the van, but if I exert force the pedal will continue to the floor. Sometimes it takes very little force to keep it pushing - sometimes there is a bit more resistance. Does this with and without the engine on.

As it goes past what I perceive to be its “normal” throw, there is a sound like pushing air/fluid into the reservoir (taking the reservoir cap off while pushing tee brake yielded a noticeable increase in volume of the shooshing sound).

I tried depressing the pedal while cracking the MC line fittings a few times (didn’t notice any air coming out).

No visible leaks - reservoir level is still good. Did a helper-based bleed after replacing reservoir grommets over fall and brakes have been good for past few months - this issue just recently started happening.

Sound like a failed MC? Thanks in advance.
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85 1.9L GL Tintop

[Ahwahnee]

Have you observed the level of fluid in the reservoir when you depress the pedal?

if it rises a bit that may indicate a fault in the MC. And yes, they can fail without leaking.

[?Waldo?]

The master cylinder is the only part of the hydraulic system that can fail without leaking externally. If it is a hydraulic issue and there are no any external leaks, then the master cylinder is the culprit.

The 'shooshing' sound might indicate a vacuum leak or failure of the brake booster. The failure of the brake booster will typically result in a very hard pedal that requires a lot of pressure for a given braking force. You may have both a vacuum and hydraulic issue at the same time.

[Salinity]

Thanks. I’ll try the MC, flush and bleed first and see if that solves the issue. Booster seems to be at least somewhat working for now (noticeable difference in force needed when engine is running vs not), but of course that doesn’t mean it’s not on its way out. I’ll report back after the new MC is in place.

I’ve read varying info on whether a bench bleed is necessary (vs doing an “installed” messy version to get the air out). I don’t have any spare brake line fittings or caps which seem necessary to do it bench style - any advice there for a first-timer?
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85 1.9L GL Tintop

[Ahwahnee]

It's been a few years since I replaced the MC but I'm quite sure I didn't bother to bench bleed it. Most likely I did use a homemade pressure bleeder though.

[vanis13]

re" bleebing MC - recently did a bunch of research on this = not needed. Installed w.o bleeding and worked fine.
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83.5 Westy with Subaru 2.5, 4 spd manual, center seat, COLD A/C on 134a!, Winter camp heated with an Espar B4 gasoline furnace

www.SuperVanagon.com - some stuff I make

[Salinity]

Thanks amigos- will bleed carefully upon installation. Hope to have the part early next week.

Cheers!
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85 1.9L GL Tintop

[vanis13]

[Wildthings]

Some background history of your problem would help. Did the problem start when you replaced some part(s)? or was it already there? Are the rear brake adjusted up properly?

[Sodo]

The master cylinder has an air-eject function at the very top of the stroke.
Push it 1inch, 20 times quickly.
I’d watch the top if the reservoir fluid for tiny bubbles rising.
Do this until no bubbles rise.

Wait a while, hours, or a day and repeat.

You can also loosen the (11mm wrench) fittings a little while holding some brake pressure and observe if bubbles come out with the leakage.

Be sure to use rags and don’t let brake fluid get on your wires and stuff.

Bleeding won’t help a faulty master cylinder but its a worthwhile step in diagnosis.
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'90 Westy EJ25, 2Peloquins, 3knobs, pressure-oiled GT mainshaft, filtered, cooled gearbox
'87 Tintop w 47k 53k, '12 SmallCar EJ25, cooled filtered gearbox
....KTMs, GasGas, SPOT mtb

[MarkWard]

There was some confusion above regarding “bench bleeding” a new master prior to install. As answered, the Vanagon master doesn’t require being bench bled prior to install and could actually make a mess during install.

I rarely have the time to test Sodo’s pump and wait method. Once installed with lines snugged and reservoir full, I crack both lines open, depress the pedal, tighten lines and let pedal return. Then repeat. Soon enough you’ll have brake fluid coming out the loosened lines. Keep a couple rags handy. One under the lines and a second for cleanup. Once you have fluid at the lines, you are ready to bleed starting at RR, LR, RF, then LF.

To answer the original question again. Yes a master cylinder can wear out internally and not leak.
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[Salinity]

Thanks for the advice all. I can confirm it does seem to have been the MC - a fairly straightforward repair (if not a tad messy). I did not bench bleed, but did the procedure outlined above and it appears to have worked to a T.

I used a $15 harbor freight bleeder pump (didn’t use the supplied “L” fittings - just some tightly fitting hose over the brake bleeder valves) with a few rounds of assisted brake pedal bleeding at the end for good measure (my assistant was a broomstick with one end wedged into the headrest).

I did find pedal travel a bit too long for my liking, which seems to be resolved after adjusting both rear (drum) brakes. Was good to get in there and get things freed up - definitely need to replace those in the near future.

Cheers!
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85 1.9L GL Tintop