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Front Drum to Disc conversion on 270?

Unread postby Dodgy71 on Tue Dec 01, 2015 1:13 pm

Does anyone know if a drum brake front end can be converted to a disc brake, perhaps by using steering knuckles, calipers, rotors, etc. from an M500 chassis? Specifically, can a 1971 M375 front end be converted to an M500 type, without changing the entire axle?
I'm guessing that master cylinders and boosters would also need to be changed since disc calipers require lower volume and higher pressure than drums, but thought I'd ask the most basic question first.
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Re: Front Drum to Disc conversion on 270?

Unread postby Acesneights on Tue Dec 01, 2015 8:58 pm

The hydravacs are probably the same. It's the master and proportioning valve that would be different afaik.
It's no big deal to swap the whole axle. 4 u bolts and two shock brackets. The 270's with front drum don't stop well at all. Borderline dangerous IMHO. I have owned both and the disc in the front makes a huge difference.
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Re: Front Drum to Disc conversion on 270?

Unread postby 77Travco on Wed Dec 02, 2015 5:42 pm

I think the hardest thing is finding the parts.
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Re: Front Drum to Disc conversion on 270?

Unread postby beemerdog on Tue Sep 20, 2016 10:06 am

Here is what I have... 72 270 with a disk brake conversion on 5k axle
all my brake lines are frozen so I will beat that horse after I finish the king pins and get the rig back on the ground. Seems this was done old school and is just a swap to disks with no true splitter though there is the 2 pot master cyl. There is what looks like the failure switch with both hydrovacs leading to it and the front and rear circuit exiting. Rear hydrovac is set to the front of the master cyl and front(forward) hydrovac is hooked to the rear port on the master cyl. Is that the way it should be? Here is what I have learned so far....
Master cyl is different
Hydrovac is different for pre 73. I have proper master cyl and hydrovac for the disk brakes
Rear brakes on pre 73 8.5k axle should be 3" shoes but on disk set up rears are 4" on 10k axle.

What best makes this conversion work? I take it I need a proper splitter. from there it should just be the same?
Where does one find the proper splitter? I have old parts books but haven't yet figured out the way to cross to new numbers.
Anyone been through this yet and have any words of wisdom or guidance?
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Re: Front Drum to Disc conversion on 270?

Unread postby Acesneights on Tue Sep 20, 2016 6:49 pm

beemerdog wrote:Here is what I have... 72 270 with a disk brake conversion on 5k axle
all my brake lines are frozen so I will beat that horse after I finish the king pins and get the rig back on the ground. Seems this was done old school and is just a swap to disks with no true splitter though there is the 2 pot master cyl. There is what looks like the failure switch (Proportioning valve but yes it kinda does that too. It cuts off the fluid to the side that looses pressure ).with both hydrovacs leading to it and the front and rear circuit exiting. Rear hydrovac is set to the front of the master cyl and front(forward) hydrovac is hooked to the rear port on the master cyl. Is that the way it should be? Here is what I have learned so far....
Master cyl is different
Hydrovac is different for pre 73. I have proper master cyl and hydrovac for the disk brakes(This is what matters most. The master cyl must have a bigger reservoir for the disc side to allow for Caliper piston travel. If they are both the same size, it is NOT the correct master for disc. Disc side should be bigger)
Rear brakes on pre 73 8.5k axle should be 3" shoes but on disk set up rears are 4" on 10k axle.(That is what my 75 has so maybe someone used a later model for parts swap)

What best makes this conversion work? I take it I need a proper splitter. from there it should just be the same?
Where does one find the proper splitter? I have old parts books but haven't yet figured out the way to cross to new numbers.(I would hope the guy switched that too, although adjustable universal ones are available from Summit and other sources.[/color]
Anyone been through this yet and have any words of wisdom or guidance?

Answered what I could in red and bold above.
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Re: Front Drum to Disc conversion on 270?

Unread postby beemerdog on Tue Sep 20, 2016 8:04 pm

Ahhh the joys of making things right


The front pot (small) on the master is hooked to the rear hydrovac and then leads to the splitter and from there to the front brakes. This I know as the front calipers bled down the front pot before I had caliper failure

Large pot (rear) of the master cyl leads to the forward hydrovac then to the splitter and on back to the rear brakes
I have 3" rear shoes (smaller gvw)

Service manual for disk brakes says front brakes should be hooked to larger rear pot on the master cyl

Since I cant vouch for whoever did the swap originally do re line the system with the front circuit to the rear pot on the master?
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Re: Front Drum to Disc conversion on 270?

Unread postby Acesneights on Wed Sep 21, 2016 9:15 pm

That from what you describe sounds wrong.
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Re: Front Drum to Disc conversion on 270?

Unread postby beemerdog on Mon Nov 07, 2016 4:42 pm

Time for an update...
Front brakes... new rotors and balanced hubs, calipers, pads, rubber hoses, master cyl. (for disk brakes), new hydrovac 73 and later, speed bleeders, cups and cones, seals and kingpins, all adding up to a bunch of cash and time. So far all seems right. They release now which wasn't happening before which may have been the cause of my problems in the first place.

Rear brakes... shoes , hardware and drums look good. New hydrovac(73 and later 15x4 shoes), cups cones and seals coming soon. Once bled and the drums reinstalled I can get Rosie to move about on her own.
So I have a disk conversion that included the larger rear drums. Holy grail or big mess?

The only thing that may not be as found on a 73 forward chassis is the proportioning valve, my guess is its the splitter for a drum brake chassis. Anyone have one? Hopefully the brakes will brake and I can move forward with this project. As I get farther into Rosie I find that the bones are pretty good however the critters made themselves at home in a big way. Enough chewed wires that she ended up not only gutted but stripped of wiring too. As winter approaches I am doing all I can to get the rolling stock rolling so she can find a better resting place than where this all started.
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