Where oh where is the trapped air? (Brakes)

[NAES]

Just took my '61 out to OCTO for its maiden voyage. Drove like a dream and all seemed well.

As I was leaving I could hear the rear brake(s) dragging but couldn't tell if it was driver or passenger side at the time. The sketchy part is the brake pedal would almost go to the floor unless I gave it a quick pump first. Mind you the brakes worked fine on the drive down which was about 30 miles or so. I checked for leaks but didn't find any so I just drove home like a grandma and kept a ton of space between me and the cars in front of me.

Here's the brake system specs

Chinese dual circuit master
Rebuilt German wheel cylinders with German kits from Bustoration
New Brazilian rear wheel cylinders
New pads
New brazilian rear soft lines
New German front soft lines
New brake springs on all 4 corners.
OG hard lines cleaned up and inspected. No evidence of leakes or corrosion.
Brake shoes are adjusted and slightly drag on all 4 corners (before the drive out to OCTO).

Bus is slammed so I have the front soft lines going to the lower wheel cylinders instead of up top. I've run this style setup for 20 years with no issues.

According to Wolfsburg West, the later dual circuit master needs to be "power bled". I bench bled the master looping the front and rear circuits back to the reservior until there were no more bubbles.

I'm about to dive back into the system tomorrow to see whats up. Any suggestions on where to look for air besides just bleeding over and over? I suspect either trapped air or a bad seal allowing fluid to bypass it.

Thanks for the help, NAES
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Barndoor Mafia
Box On Wheels
SBS #00

[Tizian]

Hi NAES,

to me it sounds like your master is gone.
In general it’s always best advice to „power bleed“ the system or however one calls it.
Personally, I always do that the old-school way, which worked fine for me in all the years. Doing this you’ll need to be cautious though: If it’s a used master avoid pressing the pedal all the way down. The piston with its gaskets could travel in an area where the cylinder already has some corrosion resulting in destroying the gaskets.

For the breaks: I would never rely on no-name products. Get yourself an ATE master. It might be made in Brazil or even in China nowadays, but still it’s a quality product the company’s reputation depends on.

Start rear right, rear left, front right, front left (for LHD vehicles).

[Okie Adam]

I am going to vote blown master cylinder. I'm not anti no name masters but it sounds to me like this one didn't hold up.
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[tisius]

I agree with the power bleeding for the dual master cylinder. I first tried the "foot-down-on-the-pedal" bleeding method, but then still air was trapped at the end of my lines for some reason. After pressurizing the brake fluid reservoir, all of the air was finally bleeding out and the brakes turned out fine.

Best of luck, I know how frustrating it can be
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[aa390392]

Let us know what you discovered...good luck..
Tomas
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[ToolBox]

Check that the shoes did not re center on the drive throwing the adjustment off.

Check that you have the pushrod to master play set correctly.

[NAES]

Used a Phoenix injector to reverse bleed the system with no change. I also readjusted the pads which were all a little loose, probably from bedding in a little bit. I like all 4 corners to drag just a touch. Pedal push rod is adjusted properly so I guess that leaves me with the Master Cylinder.

Hopefully I can get one installed before Prado on Sunday. Fingers crossed.

NAES
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Barndoor Mafia
Box On Wheels
SBS #00

[Culito]

I've done two dual circuit MC conversions and haven't had to "power bleed" either. Just standard bleeding in this order: RR, LR, RF, LF. Worked just fine.

YMMV
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'64 standard w/2.0L type 4
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[NAES]

Cool man, thats my usual method as well. I'll bench bleed the new one like normal and install it tomorrow.

I'll report back.
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Barndoor Mafia
Box On Wheels
SBS #00

[bobrk]

[earlywesty]

[BulliBill]

Yeah, I seem to remember similar advice on dual circuit brake systems that you do the fronts first then the backs? Not sure why...

Bill
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Thanks for any help!

[bobrk]

[NAES]

OK so I'm back at it with this brake issue.

I replaced the Master Cylinder with a new Brazilian unit.

Brake pads are adjusted with slight drag on all 4 corners.

They've been bled multiple times so I'm confident there isn't air in the system.

The problem is that I have pedal pressure while the bus is sitting parked. Once I start driving the pedal drops about 1/3 depth requiring me to pump the pedal once or twice to get enough pedal to "safely" stop.

To recap, the Master is new
pads are new
wheel cylinders up front are rebuilt
rears are new Brazilian
New soft lines
No leaks found anywhere.
Fluid level stays the same.

It almost feels like I should have a residual valve somewhere in the systems. If I remember correctly, there is no residual valve in the Master Cylinder.

Any help would be greatly appreciated.

NAES
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Barndoor Mafia
Box On Wheels
SBS #00

[Clara]

FWIW,
on single circuit the advice is bleed rears first, then front.
with dual, bleed fronts first, then rears.

When I have the whole system open, often it helps to go back and bleed it again the next day. Seems hard to get the last bubbles out.

I am not sure what power bleeding is, never bothered with it. I don't think it is needed. Is that another term for bench bleeding?

Once met a problem on someone else's '65 bus, which had recently had all new brakes installed. It came to me cause they lost the clutch, right as we were going to a show weekend. Lent them my 65 to go to the show that weekend, and they were amazed how good the brakes were. My 65 had good condition stock 65 bus brakes.

Turned out they had a faulty master cylinder. It had been recently purchased, but was evidently not good.
Brake going down half way sounds like one half of a dual circuit MC is iffy.
That's my 2 cents.
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[Braukuche]

If your new one has a bolt head on top of the MC you have to crack that to bleed out air. Make sure there’s some pressure on the pedal and left that way till you tighten.
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[Vanell]

You say your bus is slammed?, maybe your master cylinder reservoir is lower than a wheel cylinder, mount a bay window reservoir up higher and I bet your problem goes away, greetings from Australia, Ian

[BarryL]

Without rereading are you adjusting the rears with the E-brake fully OFF?

[NAES]

Ok. Crack the little upper bolt on the master.

Bus is on a lift when bleeding so cylinders are down low(ish)

E-brake is off.

Bleed the fronts first.

Thanks for the added tips everyone!

NAES
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Barndoor Mafia
Box On Wheels
SBS #00

[Vanell]

It’s got nothing to do with the bleeding process, when your master cylinder reservoir is lower than your wheel cylinder it will suck air in through the cap when you are driving,it’s not a sealed system and the cap breathes, that’s why you have shit brakes, just mount the reservoir inside the cabin just like a bay window is, I usually put it in the spare wheel area, as high as you can, regards Ian