Dual Reservoir Brake Master Cylinder - which outlet for F vs R?

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TheFlyingDutchman

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I haven't been able to get consistent info on this with broader web searches, so I'm posting here with the hope that there is a consistent convention within this generation of GM trucks.

Background: I have a 1993 Chevy G20 that originally came with front discs on IFS and rear drums on a FF 14-bolt. It had the primitive 1st gen ABS. The van has since undergone a solid front axle swap and a rear disc conversion. With the ABS no longer functioning due to the front axle swap, the ABS control module and its nest of plumbing has been removed with new, hard, chassis-mounted lines now running directly from the combo/prop valve to the axles.

My question pertains to which brake lines from the master cylinder and prop valve are meant to run to the front brakes and which to the rear. The ABS control module previously in place was a black box in that there was no way to know which input line was connected to which output line, and I was not able to find any schematics labeling its ports. Likewise with the master cylinder or prop valve.

My master cylinder has dual reservoirs (pic attached). The rear (i.e. closer to firewall) reservoir is larger and has a smaller 3/16" output line, which connects to the top rear port of the prop valve. The front reservoir is smaller and has a larger 1/4" output line, which connects to the top front port of the combo valve. The connections between the MC and combo valve remain as factory, but downstream of this is where I'm unclear.

I'm confused by the fact that the larger reservoir has the smaller line running from it and vice versa. I'd assume the front brakes would both warrant both the larger reservoir and larger line.

So which is which? Thanks in advance for anyone who is confident in the configuration.
 

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TheFlyingDutchman

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Rear is for front brakes. Fluid needs to displace volume for pad wear.
Thanks - that does make sense. Still don't understand why that reservoir has the smaller line (something to do with the ABS module?), but that may be neither here nor there.

Also, now it seems I'll need to keep a close eye on the smaller reservoir fluid level as it now is feeding discs as well.
 

MrHorsepowerLSx

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Larger the line the more volume and less pressure. Smaller the line more pressure less volume. Pressure is a restriction to flow. You don’t want the rear brakes having more stopping power then the front.
 

fast 99

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I would try to fit a larger reservoir. May be same as rear if enough room. Rear brakes will not need as much fluid in reserve because the piston should be smaller and normally less brake action in rear.
 

TheFlyingDutchman

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Larger the line the more volume and less pressure. Smaller the line more pressure less volume. Pressure is a restriction to flow. You don’t want the rear brakes having more stopping power then the front.
Thanks, and I agree with this all else being equal, but then you have the proportioning valve doing its thing and then different piston sizes and friction surface areas F & R, so given all else wasn't equal I wasn't going to rely on this relationship alone to determine line routing.

This was why I was hoping there would be a consistent convention - eg. the rear port on the prop valve always goes to the rear brakes (or something similar). But the larger reservoir being utilized for the disc brakes in a factory config seems sound.
 

TheFlyingDutchman

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I would try to fit a larger reservoir. May be same as rear if enough room. Rear brakes will not need as much fluid in reserve because the piston should be smaller and normally less brake action in rear.
I just installed a new MC, and my rear disc conversion kit was supposedly designed with the factory MC in mind, so I probably won't make any further changes there unless something isn't working like it should, but noted, and thanks.

Yes, front pistons are nominally larger than rears (JB7 vs JB6 calipers)
 

Keith Seymore

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I haven't been able to get consistent info on this with broader web searches, so I'm posting here with the hope that there is a consistent convention within this generation of GM trucks.

Background: I have a 1993 Chevy G20 that originally came with front discs on IFS and rear drums on a FF 14-bolt. It had the primitive 1st gen ABS. The van has since undergone a solid front axle swap and a rear disc conversion. With the ABS no longer functioning due to the front axle swap, the ABS control module and its nest of plumbing has been removed with new, hard, chassis-mounted lines now running directly from the combo/prop valve to the axles.

My question pertains to which brake lines from the master cylinder and prop valve are meant to run to the front brakes and which to the rear. The ABS control module previously in place was a black box in that there was no way to know which input line was connected to which output line, and I was not able to find any schematics labeling its ports. Likewise with the master cylinder or prop valve.

My master cylinder has dual reservoirs (pic attached). The rear (i.e. closer to firewall) reservoir is larger and has a smaller 3/16" output line, which connects to the top rear port of the prop valve. The front reservoir is smaller and has a larger 1/4" output line, which connects to the top front port of the combo valve. The connections between the MC and combo valve remain as factory, but downstream of this is where I'm unclear.

I'm confused by the fact that the larger reservoir has the smaller line running from it and vice versa. I'd assume the front brakes would both warrant both the larger reservoir and larger line.

So which is which? Thanks in advance for anyone who is confident in the configuration.
The master you have shown is for the M/L Astro/Safari van. The reason the front reservoir (for the rear system) is so small is for hood clearance, as you might imagine.

You should be able to find another master cylinder with the same characteristics (bore, outboard fittings, thread size, quick take up features or no) off a C/K truck with a better reservoir - or - better yet, maybe pop a new plastic reservoir on that same m/c base.

I had one of those on my Chevelle for a little while but I had to jack the back of the car up to top off the fluid. Eventually took it off and went back to the cast iron unit.

K
 
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MrHorsepowerLSx

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The prop valve will need to be changed from factory disc/drum to disc/disc as well. Just a reminder.
 

TheFlyingDutchman

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The prop valve will need to be changed from factory disc/drum to disc/disc as well. Just a reminder.
I wondered about that and specifically asked the manufacturer of the disc conversion kit about this, and he said I was good to go with the factory MC and prop valve. I'll look into this a little more. thx
 

TheFlyingDutchman

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The master you have shown is for the M/L Astro/Safari van. The reason the front reservoir (for the rear system) is so small is for hood clearance, as you might imagine.

You should be able to find another master cylinder with the same characteristics (bore, outboard fittings, thread size, quick take up features or no) off a C/K truck with a better reservoir - or - better yet, maybe pop a new plastic reservoir on that same m/c base.

I had one of those on my Chevelle for a little while but I had to jack the back of the car up to top off the fluid. Eventually took it off and went back to the cast iron unit.

K
yes, makes sense. the front reservoir is right up against the underside of the front of the hood as you point out.
 

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