I have a '72 W200 with power brakes. I just swapped in a front dodge axle with chevy spindles and disc brakes (2.94" diameter caliper pistons).

As expected, the pedal now has a lot more travel (~double) in it than before with drums. It definitely seems to stop better than before, although I haven't been able to lock up the front or rear (sure made everything inside the cab move around though )

Front has been bled a couple times (bench bled as well). I didn't open the rear circuit, so it should still be ok.

My dilemma is that I'm pretty sure I already have the stock 1 1/8" MC. I already PMed DrDiff, but the largest he offers is 1 1/8. He just suggested going larger.

I found newer dodges have a 1 1/4" MC I could bolt on. Would that extra 1/8" make that much of a difference? It looks like some of the chivvies used 1 1/4 with those sized calipers, so I'm SWAGing that it should work just fine? Should I try to find one even bigger than that?

Basically, I'd like a sanity check on going bigger before I spend time and money on it

Thanks,
-Dave

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Would that extra 1/8" make that much of a difference?

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Yes, but if it stops well enough to clear the seats why try and fix it? Unless the pedal is nearing the floor I would leave it. I would install a disc/drum combination valve if it were mine.

The pedal does come pretty close to the floor. Kind of like when they need bled, but mine eventually gets firm.

Forgot to mention I also have a pair of Caddy Eldorado calipers I'll swap on the rear at some point soon, which will probably make it worse still

Just trying to stop the horse trailer better

I just did a simaler swap on my 73 w200 plow truck. Used a 75 w200 dana 44 w/disc brakes. I had to use the proportioning valve off the 75 because my brakes did the same thing as yours. I left the orignal master and the booster on with no problems. May work for you

I adjusted the rear drums yesterday; the pedal is a little higher now. Still not quite how I'd like it though.

I think I'll leave it as is for now and move a little quicker on the rear disc swap. It doesn't get driven very often right now anyways.

After that I'll see how it is and get a bigger MC/prop valve if I need it.

Update:

Just got the rear disc swap done, and the pedal still isn't that great. I put plugs in both MC ports, and the pedal was hard and didn't move at all, so it should be good.

For the rear calipers, I took them off the brackets, put some wood in, and bled while the bleeder was straight up. The pedal went a little lower, but was firm (front port was still plugged).

Hooked up and bled the front brakes, pedal goes lower still, but still firm.

If I pump the brakes, it comes up just a tiny tiny bit, then settles again. I'm almost certain all of the air is out.

I haven't driven it yet, but it seemed to stop fine in the driveway, just with a firm pedal that almost goes to the floor.

1 1/4 MC

Hmmm! I just did a disc brake swap on my A100, using these calipers and a 1 1/8" master...same (or similar) problem! I'm also planning on doing the rear disc with Eldorado calipers
What P/N or application did you find for a 1 1/4"? Does it mount the same?
I'd be interested to know the answer.

Thanks,
Ran

Right now I'm looking online at rockauto. A '95 Ram 3500 shows a 1 1/4", with the same sized ports (might be swapped front to back though), and mounts via two bolts (just like my current one). It looks to me like it would bolt right in.

Even if I have a little bit of air left, the pedal still gets too close to the floor for my comfort. The more I think about it, I think going with a 1 1/4" will more or less solve the problem.

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I'm also planning on doing the rear disc with Eldorado calipers

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PM me before you start the rear disc swap and I'll give you some tips I found out the hard way

Thanks! I will definitly contact you.
Please update here when you do the MC swap, if you don't mind.
Ran

From a standpoint of pure solid geometry, that measly 1/8" increase in diameter means a respectable 23% increase in the surface area of the mc piston, which is a 23% increase in fluid volume moved for a comparable amount of piston travel.

(Yeah, I know. Pie are round, cornbread are square.)

Of course

I did find some Master Cylinders on Rockauto that were a lot cheaper than the one I listed above (one is only $22 there). Also found a 1 5/16" MC.

I'll try to go to a local place today and see how they compare to my stock one.

If I did my math correctly, the caliper pistons have 40% more surface area than the old slave cylinders. Going to 1.250" from 1.125" is an increase of ~20%. Since there are two pistons in the MC, I think the bigger MC should displace just enough additional fluid that it should just 'work'

I also found a big long thread at Pirate4x4 listing known brake/MC combos. Several with the same calipers and a 1.250" MC that reportedly worked great.

Although there are two pistons in the mc, only one of them is sending fluid to the front calipers (the other services the rear brakes).

Even so, the increase (call it ~20%, call it 23%) should be a big improvement.

Yesterday I picked up an MC39075 (for a chevy). It's $22 at Rockauto. Only problem is that it takes a short pushrod from the booster, and mine has a long one :unamused: I have applications for some other chevy ones, but I bet they'll be similar with the short pushrod.

I did find a dodge application ('98 Ram 2500, $50 at Rockauto, $100 in town) that I think takes a long pushrod. I'm going to see if I can find one in town today to look at.

I do have enough room to shorten the pushrod in the booster to fit the chevy MC and still have some threads for fine adjustment, I'd just like to not have to do that if I can avoid it.

Yeah, last night I looked at Pirate4X4 and did some searching on Google, Summit, RockAuto, and saw the 98 Ram 2500 MC. It looks like it might or might not work for me. I want to have somewhat deep pushrod hole, so this is good.
Mine is under the floor of the van, and the 1 1/8" MC's reservoir hardly fit, so this one might be a problem. I also don't have a booster, it's manual.
When I found it, I realized it was using the 2.94" calipers, so was wondering why I didn't see it before (I was looking at the earlier applications 1500 models and they were using 1 1/8").

It'll take me a little while until I secure shop time/help time with my buddy, so I can't do the swap right now. I'd love to know how it turned out.

Thanks,
Ran

Success?

I picked up the 1.250" bore 98 Dodge 2500 MC on Saturday. It was right at closing time so I just eyeballed the bolt spacing. It's narrower by 0.2" But the pushrod depth was close enough to work. The mounting holes can be hogged out enough to let it bolt up. The brake lines where also a different kind, but adapters fill in that gap.

Today I looked at a '78 Chevy K20 MC. It has a deep pushrod hole, and the mounting bolts are spaced the same. It has a 1 5/16" bore, and weighs about 10lbs It will otherwise bolt right in.

(I would prefer to find something that bolts in without modification, mostly so if I'm in the middle of nowhere and something breaks, I can just get a new one and put it in).

The 1 5/16"s will move ~27% more than the 1 1/8. I'm concerned it may be too much, but I think I'll just need to try it and see how it does.

What I really need is a catalog that lists every truck master cylinder and each dimension. Is that too much to ask