Robert Rausch

Mprmnc, I am 90% sure mine is a Trac-Lok, based on your post. I do remember some details about it. And you did an impressive job on your truck. Great work! I notice you changed out your master cylinder to a later model. Why?

Ditto that Danderson! You know some bean-counter made that decision.

mprmnc

The reason behind changing the master cylinder is twofold.

First, the '00 up calipers are two piston caliper with more piston surface area (and the pistons on the rear calipers OBVIOUSLY have more surface area than the wheel cylinders on the drums brakes, although you can't directly compare them as it's really apples and oranges.) Without changing the master cylinder it would have changed the ratio of surface area of the master cylinder to the caliper pistons and caused the pedal to be spongy. When Dodge changed to the two piston calipers they also changed to a master cylinder with a larger bore, I can't remember the sizes, but it kept the ratio similar to what it was with the single piston caliper and smaller master cylinder.

Second, rear drum brakes require very little in the way of fluid reservoir because as the shoes wear the slack is taken up by the adjusters. Once the shoes are adjusted (and if the auto adjusters would actually keep them that way) and the system bled you could effectively get away with basically NO reservoir for the rear wheel cylinders since they always return to the same position after each application. Disc brakes obviously adjust by filling the piston area with fluid as the pads wear. Hence the larger reservoir on the newer master cylinder. It may be possible to put the larger reservoir on the smaller master cylinder. When I get out today I'll check that out and see if it's possible.

I would not recommend going to the newer style master cylinder unless you swap out the front for the two piston calipers (which I HIGHLY recommend anyway.) The larger bore with smaller caliper area will likely create a VERY stiff pedal. The two piston caliper brakes also have the benefit of the slide off rotors making maintenance much easier and making the swap to the 3rd gen brakes a snap (which is the next installment, thanks to JasonblkZ06 over on Cummins Forum for the outstanding information.) Also, even though it seems ok as is, and probably is, I would be VERY aware that you have much more braking efficiency in the rear than you used to, as much or more than what you have up front now. Without changing to the two piston calipers you may want to think about an adjustable proportioning valve to the rear brakes. I'm keeping a close eye on mine since I didn't change any proportioning valves when I did mine and my rear brakes are locking before the fronts on gravel. But, it may be due to the front calipers needing replaced. They looked new when I bought the axle, but in trying to replace a broken bleeder I found that even thought the rotors and pads were quite new, the calipers were NOT and not in very good shape.

You know, I just realized that I went into a really long, drawn out explanation of the dynamics of things when I could have just said that I changed it because it had the larger reservoir for the rear discs and a larger bore because of the two piston calipers. If you are changing out your own axles I have a feeling you already understand these principles, but I think I'll leave it and hope it helps someone else that's reading it.

Robert Rausch

Thanks Mprmnc for the time you took to write that. I for one appreciate technical information--understanding the principles involved is very important to getting the performance right.

I haven't noticed any locking of the rear brakes, but I've certainly noticed vastly improved braking ability. There is absolutely no nose-diving anymore upon sudden braking, and with the old drums (which never adjusted correctly) the truck would nosedive every time I had to brake suddenly. Now I haven't had to brake suddenly on a gravel road. I guess I oughta try that and see what happens.

I used to have a '54 Chevy 3/4 ton truck that I put a '69 3/4 ton rear-end in. Drums all around. But the engine was the old 235 and the trans was a granny-gear 4-speed. If I hit the brakes hard on that one I could lock the rear brakes every time on pavement.

mprmnc

I checked out my old and new master cylinders and at first glance it does appear that the larger reservoir will fit in place of the smaller one. I don't know for sure but it appears that they have the same center-to-center dimensions.

mprmnc

Oh yeah, I forgot to ask. Just for my own information, does the Dana 70 Trac-Lok have the 2-piece case then, or the 1-piece? And I finally updated my signature.

Robert Rausch

I think it has the 2-piece, but I'm not completely sure. I only gave it a quick look-see, when I checked the guts after I bought it. I didn't really inspect it closely or understand what I was seeing. Next time I change the fluid I'll pull the diff cover and take a closer look. I'll probable do that in the next 2 weeks, as it's filled with 85W90, and I want to change it to 85W140.

Dan Turner

Looks like you deleted your signature.

So a smaller piston in the MC will create more pressure, a bigger piston at the caliper will create more pressure also, as long as the MC can deliver enough volume to the caliper to actually work.

-Dan

Robert Rausch

Dan, sometimes the signature gets included in the member's first post on a particular thread and sometimes it doesn't. I don't know why that is. If you look up a few posts, mprmnc's signature is there loud and clear.

mprmnc

No. All else being equal, a larger bore master cylinder will displace more fluid for a given amount of pedal, making the pedal harder to push and exerting less "pressure". Conversely, a smaller bore master cylinder will displace less fluid, making the pedal easier to push and providing more "pressure". I use the word pressure in parenthesis because it's not so much the amount of pressure produced as the mechanical advantage provided by the different amounts of leverage.