1976 Bus - Brakes Mysteriously Faded, Then Came Back -

[wagohn]

Hello all,
After buying our bus in April '20, we replaced the rear shoes and cylinders and bled the system. Front pads seemed fine, as did hoses. We have driven the bus many miles since then with the brakes working well. My daughter took the bus to visit a neighborhood friend and reported that the brakes faded almost all the way to the floor. She was able to pump the brakes to stop. However, after the pumping the brakes were fine again.
On her return, I took the bus for a test on the exact route, no issues and the brakes were fine there and back. I also tested the booster by pressing the pedal while starting and, as expected, the pedal dropped slightly - but settled well over half the available travel, as its always been. Fluid is full with no apparent leaks.
Any ideas why seemingly good brakes would intermittently fade?
Thanks!
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1976 VW Transporter, 2.0 FI Engine

[mike77777]

Your Master cylinder could be the problem.
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patience and putty

[kreemoweet]

[SGKent]

master cylinder
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[busdaddy]

[wagohn]

Thanks everyone - perhaps "halfway" down was too pessimistic, probably closer to 15-25% before we get pushback. Brakes have always been excellent since we refreshed the shoes/rear cylinders. Stops on a dime.

As for the original question, thank you! Makes total sense that if the seals in the MC were on their way out that the odd "no pressure" scenario would occur.

We will order a new MC as I know my limitations and don't fancy refreshing the old. Any tips for the removal/install? I see references to bench bleeding in other threads but I'm not sure what that entails. I'm assuming filling the MC with fluid before install - wouldn't the fluid just pour out when unplugging the holes to fit the pipes? I'm assuming the MC can just be swapped out without any mating to the servo issues?

Thanks!
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1976 VW Transporter, 2.0 FI Engine

[kreemoweet]

[wagohn]

[kreemoweet]

I might add to suggestions that some very experienced air-cooled vehicle buffs recommend taking apart
brand-new brake cylinders to clean out factory debris. I've never done so, and not been stung. Yet.
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[ToolBox]

[Whaanga]

I had a similar issue a couple of years ago. I had redone the front and rear brakes and felt I had a good system. I then had a workday with Colin (Itinerant Air-cooled) and when he drove my bus, he quickly identified that I had a bad master cylinder. What was strange to me is that when I applied the brakes, I felt I had good control - but I had learned to compensate by pushing the pedal down with a quick, last-minute foot move. Colin used a slower physical approach. So - I tried changing how I applied the brake and suddenly realized that I indeed had a problem. (see the write up from Colin's website: http://www.itinerant-air-cooled.com/viewtopic.php?...d1248fd48)

I was able to source a quality master cylinder and when Colin came for his next visit, we replaced it. It was a pretty do-able job and then not soon after, the servo failed. AND - so I had it rebuilt and now - I have much more confidence in my brake system.

From my experience and what others have written, it make sense to replace your Master Cylinder.
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Late 1973 Bay w/a transplanted 914 Engine

[oscarsnapkin]

[wagohn]

Thanks all, New MC should be here tomorrow. I just soaked the brake pipe nuts in PB Blaster for the night in prep.

On a side note, I'm using newly purchased Harbor Freight 6" ramps, should have bought them years ago. Drive right on, rock solid, and lots of work room. Great deal for $45 and rated for 13000 lbs.
I'm getting older and need a little more room.

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1976 VW Transporter, 2.0 FI Engine

[Abscate]

Start those brake line unions before you bolt up the MC, that lets you manipulate both to line them up. I find I can feel the threads engage when I have both in hand, vs one being bolted up.
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.ssS!

[kreemoweet]

[vwwestyman]

[airschooled]

It's been mentioned here before, but some new cylinders have the rear brake light switch mounted planar with the front switch, and that can get in the way of the handbrake cable. Good cylinders mount the switch at an angle. It's up to you if you want to try to return/exchange it, or make a chafe-guard for the cable; just look out for it!

Robbie
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[wagohn]

[wagohn]

So I got the old MC off and everything came off fairly easily. The only possible issue was removing the filler tube. There appears to be a "shouldered, black rubber, inner insert at the inlet and the shoulder area is torn up or deteriorated. How crucial is this, can I reasonably use as pictured, and in the pic do I have it the right way around?

Inner booster was dry.

Thanks for your attention.

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1976 VW Transporter, 2.0 FI Engine

[wagohn]

I decided to go with the frayed black grommet in the pic. Other than the shoulder it was complete. I've ordered a new one and will fit when it arrives.

New MC is now on and holding fluid. Next step, bleed the brakes.
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1976 VW Transporter, 2.0 FI Engine