Master cylinder question

[Johnl48]

Not to sound like a dope but my mechanic is giving me conflicting info
On a stock drum brake set up does the reservoir in the master cylinder closest to firewall go to the rear brakes or front brakes ?

[Accelo]

Reservoir location may be reversed on the 3/4T trucks.
Others will have to confirm this.

[72c20customcamper]

Originally Posted by Accelo

Reservoir location may be reversed on the 3/4T trucks.
Others will have to confirm this.

Mine is reversed but it has discs brakes . First picture is my 72 second is my 68 With a disc conversion but the lines are in the same orientation rear reservoir goes to the rear brakes

[72c20customcamper]

Picture of the factory setup rear reservoir goes to rear brakes

[Johnl48]

Service manual says front reservoir goes to front brakes but he insists the "front" reservoir is closest to firewall not front of truck and the first reservoir (closest to firewall) always goes to front brakes ?

[72c20customcamper]

He’s wrong

[Keith Seymore]

Originally Posted by Johnl48

but he insists the "front" reservoir is closest to firewall not front of truck and the first reservoir (closest to firewall) always goes to front brakes ?

So - is the tailgate on the "front" of the truck?

What does "front" mean to this guy?

K

[72c20customcamper]

Here’s a diagram may not be a C10 but GM did them all the same pertaining to drum brakes .

[72c20customcamper]

I would be looking for a new mechanic

[Johnl48]

Based on what I read the "early" gm master cylinders were front/front rear/rear so having said that will a new after market M/C retain that ?

[72c20customcamper]

Originally Posted by Johnl48

Based on what I read the "early" gm master cylinders were front/front rear/rear so having said that will a new after market M/C retain that ?

If it a direct replacement part then yes it will

[Keith Seymore]

Originally Posted by Johnl48

Based on what I read the "early" gm master cylinders were front/front rear/rear so having said that will a new after market M/C retain that ?

Normally it is the later disc/drum masters that go the other way.

The smaller reservoir in the front of the master going to the rear brakes allows for more hood clearance in the event of a sloping front end.

M/L Van shown as an extreme example.

K

[Rust_never_sleeps]

If he's been underneath, he's probably right, but just have him show you. Too many variations

Mine was rear reservoir --> rear drums, but IIUC, the valving in the distribution block(NOT a combination valve) could be set up either way.

[Rust_never_sleeps]

Originally Posted by Johnl48

Service manual says front reservoir goes to front brakes but he insists the "front" reservoir is closest to firewall not front of truck and the first reservoir (closest to firewall) always goes to front brakes ?

There are plenty of excellent mechanics with dyslexia. Their spatial reasoning is extraordinary, but the words do get tangled up sometimes

[Johnl48]

So what bought this on is i installed a 4 wheel disc brake conversion from lugnut 4x4. I mistakenly put on a disc/disc master cylinder and I have a spongy pedal. They then told me their system is designed to be used with a stock configuration, meaning a drum/drum M/C so I'm in the process of changing it back. Mechanic claimed my issue was the fact my lines were reversed. As you see in photo I also put on a proportioning valve on. Can I leave that or go back to the distribution block ?

[RichardJ]

In ALL Dual MC, the rear piston is called the Primary as it is the one that has the force applied directly to it. The forward piston is called the Secondary.

The Primary Circuit can be used for a front Disc or a Drum brake. The only requirement for a disc brake is that the reserve capacity in the reservoir is great enough that it doesn't run out of fluid when the brake pads have 50,000 miles on them. At 50,000 the reserve fluid resides behind the caliper piston.

Modern MC's usually have a single reservoir and a float switch to warn when the reservoir is low on fluid. Modern disc pads don't have the screech arm anymore to warn of worn pads. The low fluid warning does that.

Older dual MC's with two reservoirs often have one small reservoir, because drum brakes don't need much reserve fluid as the brake shoes have self adjusters.

The fluid pressure from the Primary Port and the Secondary Port IS ALLWAYS EQUAL. Unless of course there is a FAILURE.

What is important is the outlet side(s) of the COMBINATION VALVE. Inlet sides don't matter.

[Johnl48]

So with my configuration I explained how am I doing this ? Front/front rear/rear ?

[Steeveedee]

If the brakes are bled properly and the pedal is mushy, it's likely that the rear calipers haven't been adjusted enough.

[RichardJ]

Combination Valves for disc/drum brakes have three functions. Metering valve, pressure differential switch, and a proportioning valve.
Yours doesn't appear to have a metering valve, so I would say it was made for a disc/disc system. I can't see the normal rubber cap as in the cast metering valve in the first image and the aftermarket metering valve in the second image.

>>and I have a spongy pedal<<
Sounds like air in the lines.

It doesn't matter if the MC was intended for drum/drum, disc/drum or disc/disc as long as the bore size is correct for the brakes.

A distribution block, a disc/drum proportion valve or a disc/disc proportion valve or adjustable proportion valve are options used to get the correct relationship of the front brake pressure in relation to rear brake pressure.

A metering valve is used to reduce front brake pressure under light brake application at low speeds.

A proportioning valve is used to reduce rear brake pressure under heavy braking.

[Johnl48]

Originally Posted by RichardJ

Combination Valves for disc/drum brakes have three functions. Metering valve, pressure differential switch, and a proportioning valve.
Yours doesn't appear to have a metering valve, so I would say it was made for a disc/disc system. I can't see the normal rubber cap as in the cast metering valve in the first image and the aftermarket metering valve in the second image.

>>and I have a spongy pedal<<
Sounds like air in the lines.

It doesn't matter if the MC was intended for drum/drum, disc/drum or disc/disc as long as the bore size is correct for the brakes.

A distribution block, a disc/drum proportion valve or a disc/disc proportion valve or adjustable proportion valve are options used to get the correct relationship of the front brake pressure in relation to rear brake pressure.

A metering valve is used to reduce front brake pressure under light brake application at low speeds.

A proportioning valve is used to reduce rear brake pressure under heavy braking.

So to close this out are you in agreement the front reservoir (front of truck) is for front brakes ?

[RichardJ]

>>So with my configuration I explained how am I doing this ? Front/front rear/rear ?<<

It doesn't fricken matter.

If I understand what 72c20custom posted above, he has one of each for two vehicles.
If you have a combination valve with a proportioning section, that proportioning valve has to go to the rear brakes.
Not all disc/disc vehicles need a proportion valve, but a truck with a light rear end will most always need one to keep the rear tires from locking up.
Not all vehicles need a metering valve, depending on rear to front weight transfer when braking.

[Johnl48]

Originally Posted by RichardJ

>>So with my configuration I explained how am I doing this ? Front/front rear/rear ?<<

It doesn't fricken matter.

If I understand what 72c20custom posted above, he has one of each for two vehicles.
If you have a combination valve with a proportioning section, that proportioning valve has to go to the rear brakes.
Not all disc/disc vehicles need a proportion valve, but a truck with a light rear end will most always need one to keep the rear tires from locking up.
Not all vehicles need a metering valve, depending on rear to front weight transfer when braking.

No need to get testy. Just trying to get it right. Thanks everyone

[RichardJ]

>>So to close this out are you in agreement the front reservoir (front of truck) is for front brakes ?<<

NO, as in NO

In this image the brake is lightly applied. The fluid in red is under pressure. The Secondary Piston floats between the two equal pressure chambers. The outlet from the Primary Port is exactly the same as the pressure from the Secondary Port.
When the brakes are not applied, the two springs center the secondary piston between the two chambers.

It doesn't matter which port eventually feeds the rear brake circuit as long as a proportion valve, if used is in that circuit.