Not saying this is the right thing to do, but when my MC was failing and I had just seen an alternative MC from 85-96 Chevy G30, P20, and GMC, G3500, P2500, P3500 Vans and trucks, I decided to just try it. Saw it at the last GMCMI convention. It is a MC that is two stage. Kinda like the principal behind the two stage floor jacks that pump up quick until they meet resistance. This MC has two bores. The initial is a 40MM bore supplying a lot of fluid initially to bring the calipers and/or wheel cylinders fluid to get the pads/shoes out to contact. Then reverts to the main bore which is 1 1/4", the same size as the OEM MC. Brings the pedal up and keeps the MC from bottoming out with 80mm front and bigger calipers in rear. I like the feel of this with better brakes. Down side is one will have to be careful to watch the fluid level as the fluid reservoir is somewhat smaller. Also, installation must be with a metric sensitized booster as the OEM booster will not accommodate this MC. Also, needs a adjustable pushrod and some time adjusting it so it is not dragging the brakes when pedal is let up. The shape of it accommodates the shape of our hood. Again, this is just an experiment on my part that seems to be working. Also available through other parts houses. Just what I'm trying. No guarantees, not endorsing. Got information from another GMC'r who will remain silent for now. Here is the NAPA part#.

https://www.napaonline.com/en/p/NMCM2580

Pic installed

http://www.gmcmhphotos.com/photos/new-style-master-cylinder/p68374-mc-from-late-model-gm-trucks.html

Larry wrote on Mon, 13 December 2021 20:18

http://www.gmcmhphotos.com/photos/new-style-master-cylinder/p68374-mc-from-late-model-gm-trucks.html

So more volume makes a difference with larger calipers! It would be great to see what the pressures are at the calipers with what configuration. Thanks for spreading the word.

Not saying this is the right thing to do, but when my MC was failing and I had just seen an alternative MC from 85-96 Chevy G30, P20, and GMC, G3500, P2500, P3500 Vans and trucks, I decided to just try it. Saw it at the last GMCMI convention. It is a MC that is two stage. Kinda like the principal behind the two stage floor jacks that pump up quick until they meet resistance. This MC has two bores. The initial is a 40MM bore supplying a lot of fluid initially to bring the calipers and/or wheel cylinders fluid to get the pads/shoes out to contact. Then reverts to the main bore which is 1 1/4", the same size as the OEM MC. Brings the pedal up and keeps the MC from bottoming out with 80mm front and bigger calipers in rear. I like the feel of this with better brakes. Down side is one will have to be careful to watch the fluid level as the fluid reservoir is somewhat smaller. Also, installation must be with a metric sensitized booster as the OEM booster will not accommodate this MC. Also, needs a adjustable pushrod and some time adjusting it so it is not dragging the brakes when pedal is let up. The shape of it accommodates the shape of our hood. Again, this is just an experiment on my part that seems to be working. Also available through other parts houses. Just what I'm trying. No guarantees, not endorsing. Got information from another GMC'r who will remain silent for now. Here is the NAPA part#.

https://www.napaonline.com/en/p/NMCM2580

Pic installed

http://www.gmcmhphotos.com/photos/new-style-master-cylinder/p68374-mc-from-late-model-gm-trucks.html

MC?? Whatzzat?
Jim Hupy
Salem, Oregon

James Hupy wrote on Tue, 14 December 2021 23:29

MC?? Whatzzat?
Jim Hupy
Salem, Oregon

Master Cylinder

Larry wrote on Mon, 13 December 2021 20:18

Larry, I had purchased a MC prior to the Chippawa Falls Convention but didn't get it installed before having to travel to WI. I installed it when I got back and I have been pleased with how it performs. The two-stage, or fast-takeup, MC is designed to provide more volume to the calipers. As GM was trying to meet CAFE standards, they were looking for any way possible to get a gain in mpg no matter how minuscule. They changed the seals on the calipers that retracted the pads back away from the rotor so there would be no dragging. This caused a problem because it took too much fluid to get the pads back to the rotor so they designed the fast-takeup MC to "dump" more fluid into the brake line before starting to apply high pressure. There is a pre-chamber with a large bore (40mm needed in our case) prior to the normal 1 1/8" or 1 1/4" primary and secondary bores that delivers high volume through a special valve into the primary bore. As you indicated, to get the MC with 1 1/4" bore the MC needs to have a metric booster that has the seat size large enough for the MC. I found a MC, Cardone 13-1870, that has the same form factor, e.g., fits under the hood, but fits into our standard booster. I was originally told that someone tried this MC and it did not work but I thought I would give it a try. The problem is it has a 1 1/8" primary and secondary bore size. It does have the 40mm take-up bore. In my application, one-ton front-end with the larger rotors and calipers and Manny's reaction arm disc brakes in the back, this MC works better than the OEM MC I was using previously. I had plenty of stopping power before but now I have reduced my stopping distance significantly. I'm still not sure if the combination valve that I replaced many years ago has the pressure restriction that Dave Lenzi has warned us about (another project).

Like you, I reversed the front and rear lines because I felt I needed more volume to go to the rear 4 disc brakes versus the front. The fast-takeup only feeds more volume to the primary. I have not measured pressures at the calipers yet. My bleeder pressure gauge broke on me so I wasn't able to get that part done. Also, I found these MCs are a little harder to bench bleed.

This makes me wonder about a stock GMC Motorhome brake system. Could we find calipers that fit our application that has the pad pull-back seals and eliminate the drag of the pads on the rotors. Maybe gas mileage would improve??? Hmmm! I think what you did with the metric booster with the large seat so you could use the MC with 1 1/4" bores probably is the best way to go because you get more volume out of the primary and secondary chambers. I just didn't want to change out the booster. I do not take credit for this approach as it was initially tried out and tested by another GMCer.

rvanwin wrote on Tue, 14 December 2021 22:17

Larry wrote on Mon, 13 December 2021 20:18

this MC works better than the OEM MC I was using previously. I had plenty of stopping power before but now I have reduced my stopping distance significantly. I'm still not sure if the combination valve that I replaced many years ago has the pressure restriction that Dave Lenzi has warned us about (another project).

Hey Randy, I followed your post right up to the last couple of sentences. " this MC works better than the OEM MC I was using previously. I had plenty of stopping power before but now I have reduced my stopping distance significantly." It left me confused about which MC you are using now and what kind of braking you have, and what has reduced stopping power.

Larry wrote on Sat, 18 December 2021 13:14

rvanwin wrote on Tue, 14 December 2021 22:17

Larry wrote on Mon, 13 December 2021 20:18

this MC works better than the OEM MC I was using previously. I had plenty of stopping power before but now I have reduced my stopping distance significantly. I'm still not sure if the combination valve that I replaced many years ago has the pressure restriction that Dave Lenzi has warned us about (another project).

Hey Randy, I followed your post right up to the last couple of sentences. " this MC works better than the OEM MC I was using previously. I had plenty of stopping power before but now I have reduced my stopping distance significantly." It left me confused about which MC you are using now and what kind of braking you have, and what has reduced stopping power.

Larry,
Thanks for posting this.

One, thing, Randy said his new MC reduced stopping distance, not power, so that seems to make sense.

I have the DL sensitized booster; Does the 40mm/1-1/8" MC that you found fit the DL booster, and use sized such that it can use the same pushrod as the 1-1/4" MC? What was the the final pushrod length that you found worked for the 40mm/1-1/4" MC?

Larry wrote on Mon, 13 December 2021 20:18

One, thing, Randy said his new MC reduced stopping distance, not power, so that seems to make sense.

Randy said...." this MC works better than the OEM MC I was using previously. I had plenty of stopping power before but now I have reduced my stopping distance significantly."

Then you said.... "It left me confused about which MC you are using now and what kind of braking you have, and what has reduced stopping power."

Larry,

I just received the plunger/pin adjustment tool manufactured by Baer. The last MC I used Dave L's loaner tool. Is it your opinion that the plunger/pin tools are not satisfactory? I am about to install a new P30 MC this week.

Other then calling and asking Dave, is there a way to differentiate Dave's metric sensitized booster from his SAE sensitized Booster?

Thanks and Merry Christmas,
Tom K.

Quote:

Randy said...." this MC works better than the OEM MC I was using previously. I had plenty of stopping power before but now I have reduced my stopping distance significantly."

Then you said.... "It left me confused about which MC you are using now and what kind of braking you have, and what has reduced stopping power."

So, Randy now has reduced stopping distance, therefore more stopping power, so nothing has reduced stopping power. Au contraire.

I think he's using the 1-1/8" with

I have the Dave Lenzi two-port booster; I assume it's metric but I'll have to check; I didn't have to send a core. I understood that you have the same and have fitted the 1-1/4" MC. I'm asking if
1. You can share the length of the pushrod to save me some time if I go that way as well.
2. You think the 1-1/8" MC can use a pushrod of the same length. (I think you answered that they look the same but haven't installed to be sure.)

It seems like there's three options...
NAPA NMC M2580 ......44mm, 1-1/4" bore for metric
Cardone 13-1870......44mm, 1-1/8" bore for OEM
NAPA NMC M2580 ......44mm, 1-1/4" bore for metric

Did I get that right?