Rear Brake Size M300 Chassis Question

RVSTEVE · April 14, 2021, 11:00 PM

Anybody know what determines the size of the rear brake drums as there is apparently two sizes available for a 1970 M300. Either 12" or 13". My guess is the motor size determines size of brakes and drums. Seeing as I have the smallest V8 (318) for my LTP, I am assuming my rear brakes are 12". The M300 front brakes only have one size available (12"). Also wondering if the M300 with the 440 engine has 13" rear brakes. Thanks, Steve

tmsnyder · April 15, 2021, 10:14 AM

Tape measure and find out!

Oz · April 15, 2021, 09:55 PM

Here's a very handy reference chart:

http://dave78chieftain.com/FrontBrakePads.htm

It includes rear brakes as well.

RVSTEVE · April 16, 2021, 08:17 AM

Tried to measure but to much stuff in the way on the back side to get an accurate reading. Have the front wheels off so don't want to take the back wheels off at the same time but want to order shoes so they arrive when I begin the work. Steve

RVSTEVE · April 16, 2021, 08:35 AM

Thanks Oz. According to this chart it looks like the only option for the rear brakes are 13". Advance Auto says for an M300 there are 12" and 13" options for the rear brakes and the wheel cylinders are different also. According to them the M300 Rear 12" brakes have a wheel cylinder with a "1.0625 bore and the M300 rear 13" brakes have a wheel cylinder with a 1.25 bore. I looked in the service manual and parts manual but no clear distinction there either. Looks like I will just have to wait to wait till I get the rear wheels off the LTP and see exactly what it is before I order. Steve

Advance Auto page:
https://shop.advanceautoparts.com/web/SearchResults?storeId=10151&catalogId=10051&langId=-1&searchTerm=rear%20brake%20well%20cylinders&isFitted=1

Oz · April 16, 2021, 09:28 PM

Pretty silly to think they made two, different set ups for the same chassis without defining how to know for sure which one a specific vehicle has!

tmsnyder · April 17, 2021, 08:57 AM

Order both just in case and return the set you don't use. Advance won't care.

Elandan2 · April 17, 2021, 09:21 AM

Quote from: Oz on April 16, 2021, 09:28 PM
Pretty silly to think they made two, different set ups for the same chassis without defining how to know for sure which one a specific vehicle has!

A friend of mine had a Dodge Dakota that he bought new in the '80's, and it had different sized rear brakes from side to side !! The guys who put that together are probably still laughing about it.

RVSTEVE · April 17, 2021, 01:07 PM

Yes if it was just shoes I would do that if it didn't have to be special ordered. I will though be ordering, hopefully today if an when I get the drums off, shoes, wheel cylinders, brake hardware and self-adjuster kits which are all different for 12" than the 13" set-up.

I have finished the front brake system and the new master cylinder installation so I am now at the rear wheels and have them off the RV. My question now is if the rear brake drum and the rear hub are one piece? On my 1997 Ford dump truck the brake drum comes off separately with the help of a small sledge and you don't have to take the hub off to work on the brakes. Are these 1970 M300's the same or is the drum and hub one piece? They look like they are one piece.

Also are the drivers side hub bolts left handed thread like the lug bolts are on the drivers side of the RV? Thanks in advance for any Dodge wisdom. Steve

p.s. My grandfather was a tractor trailer truck driver who got the million mile accident free award and he always told me buy Dodges. Taking his advice I started out with Dodges and somehow now ended up with a bunch of Fords that have driven me crazy sometimes working on them. Sometimes Ford makes no sense. s.r.

RVSTEVE · April 17, 2021, 01:45 PM

One would think the resolution to this dilemma would be to call Dodge parts and give them the VIN number and they should be able to tell what size rear brakes are on the vehicle. unfortunately they only go back as far as 1984, so it looks like the only way to know is when I take off the rear drums. Steve

RVSTEVE · April 17, 2021, 06:21 PM

Drum and hub came off together. Drum size is 13". The lug nuts were left handed threads but the hub and the bearing retainer nut were not left handed on the drivers side. Seems illogical.

Unfortunately for me the last owner did not change his rear brakes when needed so I have one drum so far, and probably both drums, with two rows rivet gouges on the inside of the drum. I assumed because he was into going to the race car events per all the racing car decals on the LTP, he would be there type that takes good care of his vehicle, at the very least the brakes.

If I can't find the drums I guess I will have to lay a bead in each row and grind it down to make a smooth surface. Photos attached. Steve

RVSTEVE · April 18, 2021, 10:54 PM

Got the passenger side rear drum off today. Fortunately for me this time the drum on this side was in good shape (see attached photos) , the brakes shoes were new as is the wheel cylinder and brake hardware. I would surmise that that the previous owner was doing his own work and couldn't get the drum off of the other side off and just left it. I had a tough time getting it off and had to use a torch on the hub bolts to get them off without breaking them and the axle was difficult to get out also. The bolts on the passenger did not need any heat at all to get them off. I actually thought they were on to loose and the axle came right out with a couple of hits.

Been trying to locate new brake hardware and new self-adjusters for the rear drums and no one has them so I will have to reuse what I took off till I locate new stuff or make stuff work from another model. I haven't been able to locate the wheel seals either, spending a lot of time on the computer searching. Just found out that TIMKEN will make custom seals for any application so if anyone is having trouble locating oil seals for their rigs contact Timken (https://www.timken.com/wp-content/uploads/2020/02/National-Indust-Seals-Catalog_7707.pdf). Onward. Steve

RVSTEVE · April 18, 2021, 11:05 PM

Added Note: For anyone who has not had to change a tire on these old rigs that have low hanging exterior skirts around the perimeter, it would be wise to travel with two jacks to change a flat tire. One jack to get the tire off the ground and one jack to jack up the body high enough so the tires can get past the exterior skirt. I was barely able to get my dually rear tires off using two jacks.

Steve

Oz · April 19, 2021, 08:59 AM

Yep, learned about needing 2 jacks to change the rear tires when I got new tread.

tmsnyder · April 19, 2021, 11:33 AM

Oh wow, you're working off a flat bed trailer, interesting. Beats bending over !

RVSTEVE · April 20, 2021, 12:46 AM

Yeah, the trailer belongs to the storage owner. He towed the LTP in and is letting me work on it on his trailer and in his storage property. He is not charging me any rent for keeping my rig till I get it fixed and off his trailer. Perks I guess for buying the LTP from him including a no interest payment plan. How could I not buy it? Steve

RVSTEVE · April 20, 2021, 12:58 AM

Was able to order all the wheel seals, front and rear inner and outer, at NAPA at the store. Online said they only had seals for a 70 M300 with 4,800lb differential or 8.75 ring gear. This M300 rear is 3,800lbs and the ring gear is 10.50. I then checked at the store and they were able to order the seals with no problem while their online service said they didn't have them. Moral of the story is always check with the store as well as online.

Steve

Oz · April 20, 2021, 02:31 PM

That's good news.

RVSTEVE · April 23, 2021, 11:02 PM

Correction: The Spicer 70 rear differential in my 70 Dodge M300 is 7500 lb. capacity, not 3800 lbs. as I mistakenly stated previously. The front axle has the 3800 lb. capacity.
BTW in my searches for a rear drum for my rig, I found new rear drums for 72 and later are available for M300 motorhomes. Send me a PM if you want that info. Steve

Oz · April 25, 2021, 09:07 AM

Would you just post that info, please. Easier for people who may need it.

RVSTEVE · April 25, 2021, 09:59 AM

Oz, I did post that info and it never made it on the site. I thought maybe It looked like I was trying to sell products and you edited it out. I will post it again. Steve

RVSTEVE · April 25, 2021, 10:34 AM

Sources for new rear brake drums for 1972 and later Dodge M300 motorhomes:

1- 1972 Dodge M300: https://www.stockwiseauto.com/centri...YaAoTwEALw_wcB ($54.72)

2- 1974 Dodge M300: https://www.partsgeek.com/gbproducts...UaApDTEALw_wcB ($63.71)

Both these drums may possibly fit other years post 1971 besides 1972 and 1974. Steve

RVSTEVE · April 25, 2021, 11:31 PM

Does anyone know what the center hole dimension is for the 1970 Dodge M300 rear brake drum?

Does the 1972 or 1974 Dodge M300 have a Spicer 70 rear differential like the 1970 Dodge M300?

I have not separated the hub and drum yet till I know I have another brake drum available and can't tell what the center dimension is without separating it. I need to know the center hole dimension to locate another matching brake drum.

Thanks, Steve

Oz · April 29, 2021, 01:34 PM

Thanks for posting the links. Nope, I didn't edit them out.

MinnieWinnie_75 · November 14, 2021, 08:16 PM

Any idea if they make a disc brake conversion kit for the m300 chassis??