Master cylinder question

[Johnl48]

I'm starting to work on the brake lines. I got pre bent from inline tube, nothing fits right but that's another story. Anyway I did a 4 wheel disc conversion, my master cylinder has a large reservoir and a small one so just want to confirm the large reservoir near the rear would be for the front brakes, correct ?
My old MC had equal size reservoirs

[dmjlambert]

Yes, the large reservoir is for the disc brakes in a front-disc-rear-drum setup. Normally on a C-10 the large reservoir is in toward the front of the truck, and on a C-20 the large reservoir is the one closest to the firewall and the tubing crosses each other on the way to the brakes.

Edit: Inline Tube told me they don't sell tubing for conversions.

Edit: I just re-read your post, if you have disc brakes on all 4 wheels then I suppose you're needing equal size reservoirs for front and back.

[Johnl48]

Originally Posted by dmjlambert

Yes, the large reservoir is for the disc brakes in a front-disc-rear-drum setup. Normally on a C-10 the large reservoir is in toward the front of the truck, and on a C-20 the large reservoir is the one closest to the firewall and the tubing crosses each other on the way to the brakes.

Edit: Inline Tube told me they don't sell tubing for conversions.

Edit: I just re-read your post, if you have disc brakes on all 4 wheels then I suppose you're needing equal size reservoirs for front and back.

I purchased the lines from GMC Paul's, I told him what I had and he said this would work. No worries on that tho I know it's not stock I can make some bends. As for MC I'm not sure. The original had 2 equal reservoirs for 4 wheel drums. Where I purchased it they said this is for disc brakes but that doesn't mean much I guess

[dmjlambert]

Yes it is for what most people call disc brakes, which means disc in front and drum on rear. You probably need to call a vendor who specializes in helping with rear disc brake conversions and they will sell you a master cylinder with equal size and large reservoirs, probably for a year of Corvette that had 4 wheel disc setup.

[Johnl48]

I checked lug nut where I got the conversion kits from.

[geezer#99]

Their description explains the lack of a proportioning valve.
You have a distribution block that’s used for drum/drum only.

[Johnl48]

Originally Posted by geezer#99

Their description explains the lack of a proportioning valve.
You have a distribution block that’s used for drum/drum only.

They just emailed me. They said I would need one for 4 wheel disc brakes with both reservoirs the same size. They suggested one from a 75 k20 ?
As for the distribution block you mentioned, that's a proportioning valve for disc brakes.
Apparently there is a check valve in a drum brake mc to keep slight pressure on the drums

[geezer#99]

I stand corrected!
The check valve is called a residual check valve.

You might need a m/c from a 4 wheel disc corvette.

[Johnl48]

I looked up a 75 k20 and it has drum brakes. Can anyone recommend a 4 wheel disc MC ? That will work

[geezer#99]

Like this.


https://www.corvettepartsworldwide.c..._w_p/49619.htm

[Johnl48]

How about this one ? A bit more budget friendly

https://www.inlinetube.com/products/mpl506

[geezer#99]

Likely the same one.
The corvette one is likely casting code correct.

[jocko]

If a vendor is recommending a 75 K20 mc for a 4-wheel disk system they just sold you, I’d probably not ask that vendor any more technical questions.

[dmjlambert]

Originally Posted by jocko

If a vendor is recommending a 75 K20 mc for a 4-wheel disk system they just sold you, I’d probably not ask that vendor any more technical questions.

Funny and sad at the same time. Good idea.

[PbFut]

Why would you need same size reservoir on each circuit? It is a fluid reserve to take up space in the caliper as the pad wares down. It is not likely the caliper piston is same size front to rear. The master piston should be correctly sized. Reservoir is not reletive to caliper performance. Each circuit relative to the calipers is sized to provide the correct volume of fluid to proportionally provide the needed pressure of the piston against the pad.

[Johnl48]

Based on this shouldn't I just get a stock MC for my 70 ? Or just use what I have and see what happens

[geezer#99]

Look on here in the tech talk section.
Might find some info there.


https://www.mpbrakes.com/

[RichardJ]

PbFut posted the correct information about reservoir size.

On the old drum brakes the reservoir did not have to be very large. As the brake shoes wore down the self adjusters compensated for the wear as the shoes got thinner over 30-50,000 miles.
Disc calipers do not have a mechanical method to adjust for wear of the pads. When the disc pads are new there is very little fluid stored behind the caliper pistons. At 50,000 miles, the stationary volume of fluid stored behind the caliper piston increases.

Hydraulic brake systems, drum or disc do not transfer a significant volume of fluid through the brake lines as you step on the brake pedal. You're transferring pressure from one end of the brake line to the other.

Most modern master cylinders use a single reservoir to feed the primary and the secondary pistons in the MC. Most of these reservoirs have a level sensor, which tells you the brake fluid in your 50,000 mile brakes now lives behind the caliper pistons.

[Johnl48]

Originally Posted by RichardJ

PbFut posted the correct information about reservoir size.

On the old drum brakes the reservoir did not have to be very large. As the brake shoes wore down the self adjusters compensated for the wear as the shoes got thinner over 30-50,000 miles.
Disc calipers do not have a mechanical method to adjust for wear of the pads. When the disc pads are new there is very little fluid stored behind the caliper pistons. At 50,000 miles, the stationary volume of fluid stored behind the caliper piston increases.

Hydraulic brake systems, drum or disc do not transfer a significant volume of fluid through the brake lines as you step on the brake pedal. You're transferring pressure from one end of the brake line to the other.

Most modern master cylinders use a single reservoir to feed the primary and the secondary pistons in the MC. Most of these reservoirs have a level sensor, which tells you the brake fluid in your 50,000 mile brakes now lives behind the caliper pistons.

So basically if I check the fluid on a regular bases I can stick with this mc ? Or do I need the dual reservoirs the same

[Rust_never_sleeps]

https://mbmbrakes.com/which-master-cylinder-is-for-me/

I have found the mbm tech info on their site to be really helpful and clear, so maybe check out the "tech" and "downloads" sections there^^

[Johnl48]

Originally Posted by Rust_never_sleeps

https://mbmbrakes.com/which-master-cylinder-is-for-me/

I have found the mbm tech info on their site to be really helpful and clear, so maybe check out the "tech" and "downloads" sections there^^

Only snafu with this is manufacturer of the kit says calipers were made to run off the original mc for three drums

[Rust_never_sleeps]

I feel your pain on having to bend up "pre-bent" hard lines, though there's just the faintest chance that that's entirely my fault. ;-)

[Sheepdip]

I converted Otis to front disc brakes a while back I used a 71/72 C20/30 master cylinder. The rear pot, (front disc) is slightly larger, not by much but it is bigger. The front pot (rear drums) is and would be plenty big to provide an ample fluid reservoir for a disc set up.

[Johnl48]

Just ordered this one

https://piratejack.net/mc309-gm-1967...ster-cylinder/

[geezer#99]

If you looked at the info from the link in post #20 you’d likely see that the one you ordered won’t work.
You need a dual disc master. Like from a dual disc corvette.