> 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, 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.
--
Randy & Margie
'77 Eleganza II '403'
Battlefield, MO
