Reverse Filling Juice Rear Brake?

Has anyone tried this?

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Mongrel505558
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Reverse Filling Juice Rear Brake?

#1

Post by Mongrel505558 »

For my bikes with disk brakes I like to reverse fill with a kit I have that attaches a hose to the bleeder screw and pumps the brake fluid up to the master cylinder. I works great and I don't need to spend much time bleeding. In fact sometimes I don't need to bleed at all. I'm wondering if I can do this with my '68 juice rear brake. I know the master cylinder has an extra thimble-shaped part for use with the rear drum that you would remove if using a disk caliper. I believe it's function is to allow the wheel cylinder to return properly when the brake pedal is released. Would this part impede reverse filling a juice system? I can try it and find out for myself, but I generally like to avoid making a mess with brake fluid.
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Re: Reverse Filling Juice Rear Brake?

#2

Post by RooDog »

Jim...
The piece you speak of is the residual pressure valve. Its purpose with drum brakes is to maintain just enough hydraulic pressure in the system to keep the cups in the wheel cylinder expanded. On cars it works at about 3-5#. This is an unnecessary feature for disc brakes as their O ring seals are self supporting, and any residual pressure would result in the caliper's pistons not relaxing and the pads to drag. OK?
It also acts as a check valve when bleeding a drum system, so reversing the flow of the fluid may, or may not, work, depending on how much pressure your bleeder can muster.....
Let us know how it works out for you. But I have found that the standard pump & hold method works well for the rear brake, but not so for the gravity affected front....

....RooDog....

PS: Ignore all this prattle if dealing with mechanical brakes....
Mongrel505558
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Re: Reverse Filling Juice Rear Brake?

#3

Post by Mongrel505558 »

RooDog wrote: Thu Apr 14, 2022 4:56 pm Jim...
The piece you speak of is the residual pressure valve. Its purpose with drum brakes is to maintain just enough hydraulic pressure in the system to keep the cups in the wheel cylinder expanded. On cars it works at about 3-5#. This is an unnecessary feature for disc brakes as their O ring seals are self supporting, and any residual pressure would result in the caliper's pistons not relaxing and the pads to drag. OK?
It also acts as a check valve when bleeding a drum system, so reversing the flow of the fluid may, or may not, work, depending on how much pressure your bleeder can muster.....
Let us know how it works out for you. But I have found that the standard pump & hold method works well for the rear brake, but not so for the gravity affected front....

....RooDog....

PS: Ignore all this prattle if dealing with mechanical brakes....
Thanks for the explanation of the residual pressure valve. If it also acts as a check valve then I don't see reverse filling as a practical method. I'll just take the hose off of the pump and let it run from the bleeder screw into a jar with some brake fluid in it and fill the system from the master cylinder.

BTW, the working pressure of that valve is probably 3 -5 psi, since both the master cylinder and wheel cylinder are the same part numbers as used in several cars.
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Re: Reverse Filling Juice Rear Brake?

#4

Post by RUBONE »

The reverse fill process is fine, no issues with valves or not in the mc. I just use a piece of line and a glass jar for a reverse siphon effect. Bubbles come out in the jar, fluid sucks back in when the pedal is released.
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Re: Reverse Filling Juice Rear Brake?

#5

Post by RooDog »

I like to call it a "hat valve" because it is shaped like an old Derby, Bowler, hat. It is held against the rubber covered metal washer by the spring inside the MC. Once the pedal pressure is released, the wheel cylinder's pressure, provided by the brake shoe springs, forces the valve off its seat and the fluid back flows into the MC. It's complicated, but actually quite simple once one realizes what's going on in there.....
On my rear disc brake, I dismounted the caliper, laid it on the floor with the bleeder screw open, and let gravity do its job. I watched it quickly bleed out while sipping a cold one....
....RooDog.....
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Mongrel505558
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Re: Reverse Filling Juice Rear Brake?

#6

Post by Mongrel505558 »

RooDog wrote: Thu Apr 14, 2022 5:39 pm I like to call it a "hat valve" because it is shaped like an old Derby, Bowler, hat. It is held against the rubber covered metal washer by the spring inside the MC. Once the pedal pressure is released, the wheel cylinder's pressure, provided by the brake shoe springs, forces the valve off its seat and the fluid back flows into the MC. It's complicated, but actually quite simple once one realizes what's going on in there.....
On my rear disc brake, I dismounted the caliper, laid it on the floor with the bleeder screw open, and let gravity do its job. I watched it quickly bleed out while sipping a cold one....
....RooDog.....
DSC03330.JPG
I don't really have that option with my '68.
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Re: Reverse Filling Juice Rear Brake?

#7

Post by RooDog »

True....
Just saying that a rear disc can be easily bled without any special apparatus....
Also it is on the same side as the MC, so one can stretch out and do both the pump and crack the breeder ones self. The drum is on the other side, and a helped would make things easier....
But, ain't it all just too much fun?
....RD....
Mongrel505558
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Re: Reverse Filling Juice Rear Brake?

#8

Post by Mongrel505558 »

RooDog wrote: Thu Apr 14, 2022 5:58 pm True....
Just saying that a rear disc can be easily bled without any special apparatus....
Also it is on the same side as the MC, so one can stretch out and do both the pump and crack the breeder ones self. The drum is on the other side, and a helped would make things easier....
But, ain't it all just too much fun?
....RD....
For the fun factor it's right up there with removing a 4-speed transmission main seal. A couple of my least favorite motorcycle maintenance activities. I agree with you about bleeding a rear caliper. My GMA on my pan is easily bled. Everything on that bike is easy to work on because there's so little there. Changed a clutch cable on that bike in about 10 minutes yesterday. The open belt primary allows very easy access.
Mongrel505558
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Re: Reverse Filling Juice Rear Brake?

#9

Post by Mongrel505558 »

Tried reverse bleeding the juice brake for a second until I realized there's what I think is a speed bleeder on the wheel cylinder. Not getting anything past that, at least from the outside. Did it the conventional way. Went pretty smoothly.
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