Replacing Drum Brake Axles with Disc Brake axles

[rcaircraftnut]

How about the axle on the 72 M375/400. I have drum brakes and need to either replace them or repair them. Id love to have disc brakes in the front. Will the disc brakes from the later like 73 M400 be used on my early axle? I realize I will need the disc master and prop valve thanks.

[DaveVA78Chieftain]

rv,
The parts manual appears to list the same p/n for front axel I-beam for both M375 and RM350 (redesignated to M400 in 75).  The sore spot is that a M375 has 14 1/8" x 3" rear brakes where the RM350 has 12" x 3" rear brakes.  M300 only had single piston disc calipers.  M400 amd larger has dual piston calipers.  Additionally, the M400 migrated to Hydroboost in 74.  The differences just mean the master cylnder/boosters may not be the correct size for the rear wheel cylinders (i.e a M400 master designed for 12x3 rear brakes trying to drive a 14 1/8 X3 rear brake.

Was not a M300 issue as they had 12x3 rear with either drum or disc.

Dave

[rcaircraftnut]

Ok thanks.

[Mike Simon]

I have been considering doing this updrade on my 72 M-375.  Wouldn't the displacement of the cylinders/calipers be more important to match than the shoe size?  I do understand that the larger shoe has more friction, but the dual piston has less pressure using the same volume.  What if the 72 twin plex dual rear cylinders and a 73 dual calipers were close in displacement?  And if so, use a new matched master/hydroboost, properly metered with residuals for the rear, such as a Bosch Hydro-Max sytem.  I would really like some input on these thoughts.   

[jkilbert]

we all know the advantages of front discs, has anyone though about a salvage yard Dana 80 with rear discs? this is what Dodge put some of the 3500 series Cummins powered P/U's and cab/chassis for dumps.

[polarman]

I have been considering a disc swap in my 73 D20. That's the M300 chassis. I assumed that the parts would be so hard to find and expensive that it would not be doable on my budget. Is this something that is relatively easy to do? Are the parts needed still available? I have a front wheel that locked up as soon as I got home from my last outing. I assume that it dropped some hardware or shot a puck out of the wheel cylinder.I'm sure that it won't be easy to get apart. I have had lots of vehicles with drum brakes, but nothing this heavy. I'll admit that I get a little spooked out on long steep downhills that end with a stop sign, so I need to freshen up the stock brakes or do a disc conversion. I hope this topic draws in more information to help me decide which way to go.

[Oz]

Tom D'Epiro did a front axle swap.  Try the message search, I'm not sure if he covered it completely but yes, it is doable!  You can also contact him, he's a Dodge Guru!

[Slantsixness]

Let me start this by saying I HATE FRONT DRUM BRAKES. And, if you look in the Archives, you might find where I've explained this before.
This is a little rusty, since its been close to 10 years since I did my conversion... and the winnie still stops like a compact car (maybe a little extra pedal effort...)

on a D20 (M-300) disc brake swap is not hard. Rear brakes are identical through the 1990's (m-300, RM-350, D-350, D-400, D-300, D3500. they all have the same 12X3 shoes, and the same single wheel cylinders.)

You want a proportioning valve from an RM 350, 1975 up. This has the right flow rate and mounts in the same place as the original 71-73 drum brake proportioning valve. (but, the valve on the later chassis was not mounted in the same location. don't expect to find it in the same place as yours in a junkyard, but using a Junkyard Prop valve is just fine, you can clean it and test it easy enough and they almost never fail. DO NOT USE the original Drum brake proportioning valve. It will not work! hydraulic flow rate in the front brakes is way different! (if you were to do that... surprise!... the only brakes you'll have would be the rear!)

Ok.. nitty gritty.... you didn't ask for this much info.
Find Spindle hardware, Rotors, caliper mounting plates (where your backing plate is for drum brakes), hardware and calipers on an RM-350 chassis Basically the whole 9 yards, but not the axle itself or spindles, they're the same! use the calipers as cores and rebuild, or buy rebuilt (they're cheap, by the way.) You will have to buy a Tap for one hole in the spindle. (I'll have to look up the size (7/8-32?), but its simple to do)
Remove everything down to the spindles on your front axle. Tap the one hole, reassemble (new seals, bearings, bearing races (not expensive).
You will need to replace the M300 Drum brake Master cylinder with an RM350 master cylinder. (Can't find one? look up a 1978 B350 Sportsman dodge Van. It's the same... as well as almost all other Dodge 1 ton trucks! they all bolt right up to that giant brake booster. (helps to have the one off the stripped RM350 chassis though, just for the warm fuzzy feeling that its the same!)
Remove and discard all brake lines, hoses, all of it. use new lines only, 1977-1990 B350/B3500 Van Front flexible brake lines are a direct fit (Pickup truck hoses are 3" too short), but there is a crimped=on bracket that will have to be removed, or just leave it on, like I did  (just go for the fit on the caliper hydraulic line and the length, the brake line fitting size is going to be different. that's why you replace the lines... and those are  cheap too! It doesn't hurt anything, doesn't get in the way of steering or flexibility.) Rear Flexible brake line is still available (NAPA). Tell them it's off a Dodge 1 ton pickup, say 1978. same diameter, direct fit.
there used to be a place in Boston that supported the brake parts you need, I have no idea if he's still around, but at one time, he took a member here for a really bad and unfortunate business deal on a Hydrovac rebuild and runaround.
Oh... your booster is probably fine. change the air filter, change the dry rotted vacuum lines and it should work like new... and of course work with the disc brakes too.
Everything bolts up like it was made to fit. Well, it is made to fit! wheels will bolt on with the correct Keyed and conical offset lugs, and the best thing of all? When you hit the brakes and find out there is no brake fade, pulling, or grabbing. Your Winnie just comes to a rather Quick STOP! on demand! no adjusting needed, no leaky front wheel cylinders.. nothing. Just Stopping like it should have when it was new (but never did).

my total expense (10 years ago) on the brakes, bearings, seals, everything was about $700, including the "junkyard" parts. but everything including the rear brakes was totally new. Labor was free, I did it all in about 3 days... and I'm lazy.

Hope this helps!

Tom

P.S. Here's the link to the info I posted when I first considered this swap:.
http://www.classicwinnebagos.com/forum/index.php/topic,1036.msg1055.html#msg1055

[DaveVA78Chieftain]

A couple of things to note:

1) In the 74-75 manual supplement, the 73 and later RM350 chassis was redesignated as M400.   Creates a lot of confusion at times.  From what I have seen though, a lot of the aftermarket supply people retained the use of RM350.  So, use both designations when looking for parts.
i.e: You will not find listings for M400 rotors but will find listings for RM350. 

2)  Because Tom's upgrade is based on the Hydro-Vac configuration, make sure a replacement master cylinder is from a 73-75 Hydro-Vac setup.  Dodge started converting to Hydro-boost brakes in 75 (different MC).

Dave

[Slantsixness]

I have to Correct Dave's second note:
M300, RM300, RM350 under 24 foot overall length with Bendix brakes all use MasterVac Boosters. I do not have HydroVAC boosters. The primary difference between the M300 and M-375 is the inclusion of the dual Hydrovac Boosters. While even Dodge states that the RM350 was later designated as the new M-400 (as was the M375), without the dual hydrovac boosters, and the single MasterVac, the M400 and RM350 would be virtually identical. later chassis designated as "M300" ARE NOT the same as the pre-1974 M300 chassis. later M300's are the Class C (Van/ Kary Van) chassis have independent front suspension and have hydroboost brakes (a different animal altogether).
It all has to do with overall length and weight of the loaded Chassis. The rule of thumb is, "less than 24 ft, never seems to have HydroVac." D-18 to D24 models will typically be old M/RM300 or RM350 chassis, Which does not mean that there aren't plenty of D24's out there with HydroVac boosters, it really depended on the overall loaded weight of the finished product or the way the Coach manufacturer specified the chassis.  which is better? well. MasterVac is cheaper... I've had either, none of them seem to stop any different, except when they have Drum brakes. (did I mention I hate drum front brakes?)
If you look at a junker and see a MasterVac, with Disc brakes, you can use them on ANY D18, D20 or D22 that has MasterVac and Drum front Brakes. It's almost a bolt in swap.

Parts on MasterVac bendix systems (like the master cylinder) are very common and interchangeable throughout the Dodge Truck lineup through about 1993. most parts, even the shoes, pads and hardware are common with 1 ton dually trucks (D300/350/3500, and Van/KaryVan chassis (RM350/B350/B300/B3500). Just be careful you're replacing apples with apples, and it'll work.

[skwerlz]

I am considering a disc conversion as I have no brake pressure at all despite no visible leaks.
I posted images of my rig so I can be sure I am hunting for the right parts here https://www.classicwinnebagos.com/forum/index.php?topic=16518.0

Any help would be greatly appreciated.

[Oz]

Call Geoff at www.alretta.com about the parts and problem. You don't have to have a visible leak to not have pressure.
A disc conversion is quite involved.
Do a message search on no pressure.
Weigh your options.