Brakes not working after m/c change

[Akasharaja]

Hi all, the brakes recently failed on my ‘72 Bay. There were no leaks bad the shoes were properly adjusted. Having asked around I concluded it was the master cylinder. They aren’t expensive, so I replaced it. But when bleeding the brakes with the new cylinder in place, the fluid only moves through very feebly, and I can hear bubbling from the reservoir. Still no leaks, though Any ideas on what’s wrong? Thanks!

[Wildthings]

Did you bench bleed the master? You need to check and adjust the pushrod length when installing a new master as well.

[Akasharaja]

I did not bench bleed the master, no. Though I’ve replaced it before and didn’t need to then.

[bsairhead]

Soft lines swelled shut.

[Abscate]

You need to bench bleed to not only help bleeed the system, but confirm you have a working new part. Welcome to the Scatty Supply Chain of 2023.
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[Bonesberg55]

Bench bleeding the MC is necessary if you don't want to be bleeding the system for hours.

[Akasharaja]

Gotcha. I’ll do that thing and hope I haven’t got a duff part. Is there a specific bleed kit for master cylinders?

[kreemoweet]

I've never had to "bench bleed" any m/c. My bus brake system bleeds fine. If you keep the bus level, the m/c will bleed itself, as the air in it rises by buoyancy (i.e. reverse gravity) up into the reservoir. It takes a minute or two. It is also perfectly normal for fluid to just barely dribble out of the bleeder valves when beginning the bleed procedure, because the lines are full of air and the pedal movement produces squat for pressure in air.

This all assumes you have functional brake parts of course. It is not rare for m/c's to be bad new out of the box from your crappy vw parts vendor.
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[wagohn]

[Akasharaja]

A couple of you have mentioned Push rod adjustment. Is there a rule of thumb to get the push rod length adjusted for a new mc?

[wagohn]

[Akasharaja]

I’ve now picked up a vacuum kit, which also says I should bench bleed the master, though I can’t really think of a sound physical reason why I would need to do so. Surely the vacuum will just draw all the air out of the cylinder chamber and through the brake line, to be replenished with fluid from the reservoir?

[W1K1]

[chuckpolzin]

use a power bleeder system and save yourself the headache, i tried to do it with the crappy little suction harbor freight deal and it was a pain, the power bleeder worked amazingly well and will eliminate any need to "bench bleed" the system. The standard power bleeder , i forgot the name brand, does not come with the correct resevoir cap so you will need to order the one that will fit the VW rez.

i believe the brand is motive but cant remember, nor can i remember the part number that fits the vw resevoir.

[raygreenwood]

Bench bleeding works.....as a concept....just not on the workbench.

As mentioned, there is just no way to reasonably keep it from getting messy and getting it into the bus (or any car for that matter) without losing most of the fluid.

I find a version of bench bleeding...or "pre-bleeding" really helps especially in type 3 and 4 car master cylinders because some makes of them can have an odd port to seal geometry issue inside that traps air.

Fill the MC with a little fluid to wet everything. Cycle it. Drain excess and wipe off.
Do not open any bleeder valves yet.
Install MC and pump the pedal hard and fast downward and very slow upward. It's easier at this point to pump the pedal with your hand.

Push it down to bottom of stroke and block it there with a rod. Get kut with your flare wrench and crack each hard line at the MC just until you hear or see air/fluid escape. Repeat a couple times. Keep a rag handy. Wash down when done.

This is essentially the same as bench bleeding but actually works. Bleed with normal method or with pressure bleeder from here. Ray

[75 Westy FI noob]

Did you change the shoes on the rear drums by any chance ?

[75 Westy FI noob]

i worked on my brakes for nearly a month without getting any pedal at all.
Bleeding, bleeding again, moving bus, leveling bus, bleeding again, reversing , leveling, bleeding.. vacuum.. pedal pump partner.. bleeding...


in the end, the drum shoes on the rear were not adjusted properly.. allowing too much travel in the cylinder... not enough pressure to activate the rear or the front.

tightened them sob's all the way, backed it off 5 clicks.. now the brakes are solid.

[Akasharaja]

Thanks for all your help. In the end it was down to pumping the pedal and waiting and pumping the pedal again till the air bubbles had stopped rising up into the reservoir and using the pedal method for each brake in turn. Worked absolutely fine.