Disc to Drum conversion problems...hear me out

[BeerM3]

Okay, I'm in need of help. After 2 months of monkeying with the brake system on my 1/76 I think I finally figured out what I did wrong and I'm hoping someone here can confirm.

Nobody in their right mind would get rid of a front disc axle and replace it with drum, but that's what the PO did and at the time I was too naive about FJ40 year differences to know that a 1/76 should've had disc, proportioning valve, etc. In fact the bracket that held the proportioning valve is still there. After a few months of tooling around town with decent brakes it started to get mushy and I determined the MC needed to be changed out, which was correct.

Here is what I started with.

And it was definitely starting to go south

So two months later with a new drum booster from CityRacer, MC from Napa, all soft lines replaced with OEM, rear wheel cylinders replaced with new from CruiserOutfitters, and too much DOT 3 I still wasn't getting a good pedal. Bleeding was done methodically per Coolerman's tutorial. After more frustration I discovered one of the front cylinders was frozen shut, so that wheel got two new cylinders from Napa.

Before someone jumps my case about replacing all cylinders per axle at the same time; you're right. But I have a salvaged '76 front axle that will be rebuilt in the next year or two so right now I just need to get by. The front driver's side cylinders were replaced fairly recently by the PO.

 

[BeerM3]

So I had an epiphany today and here's where I think I went wrong. The new MC is NMC M2284 from Napa. It says it fits my '76 and it's identical to the one that was removed. EXCEPT...this new one would not have a residual valve on the front line, correct? I'm guessing that when they swapped out for a drum axle on the front, they added one so that both the front and back have 10# of working pressure and won't bleed back into the reservoir? Right now, if I'm only operating the front cylinders with 2-3# of pressure needed for disc, that would explain the slooooow crawl to a stop that I'm experiencing.

Naturally I tossed the old MC. If I'm right, is it even possible to buy just the valve from a parts store and drop it in, or would that need to be scrounged from salvage? Or at this point should I just say @$%^ it and but a 1/75-9/75 MC and sell it when it's time to "upgrade" back to disc?

Any help or advice is much appreciated.

 

[Casey E]

Upgrade front axle too disc brakes, problem solved.

 

[BeerM3]

Here's what it's looking like at the moment

And as I'm typing this, I just remembered there's a old MC off a '72 that was in a box of parts that came with a parted out body I bought a while back....I wonder....

 

[BeerM3]

Upgrade front axle too disc brakes, problem solved.

That's the plan and probably this winter's project, but for now I just want to get her back on the road.

 

[Casey E]

The residual valve should be inside the front brake port. I would take it out and see if it fits inside the one you have on your rig.

 

[Casey E]

Actually either port should have it on that MC if it's an early drum brake one.

 

[Casey E]

You could also order the proporting valve from cityracer and run it on the front then switch too the back brakes once you upgrade to disk brakes.

 

[thebigredrocker]

Quickest fix might be, slip a residual valve on the master. Not sure if anyone makes a residual valve in -4 AN thread. If so, I know an adapter exists that will screw on your stock 10mm line nut. It is 10mm female to male -4 AN. I’d have do some research to figure out a part number.

 

[pb4ugo]

It's been decades since I swapped out drum to disc brakes, I seem to recall the drum brake mc was a 1" bore and the 76 disc mc was a 7/8" bore. I think the bore size is marked on the side of the mc. Maybe someone else can expand on this subject.

 

[BeerM3]

Actually either port should have it on that MC if it's an early drum brake one.

It's from an all drum '72. I'm going to do some exploratory surgery on it tonight and see if the brake line adapter thread is the same size as the one I should already have on the back line.

It's been decades since I swapped out drum to disc brakes, I seem to recall the drum brake mc was a 1" bore and the 76 disc mc was a 7/8" bore. I think the bore size is marked on the side of the mc. Maybe someone else can expand on this subject.

I know the one I got from Napa lists a 7/8" bore.

Quickest fix might be, slip a residual valve on the master. Not sure if anyone makes a residual valve in -4 AN thread. If so, I know an adapter exists that will screw on your stock 10mm line nut. It is 10mm female to male -4 AN. I’d have do some research to figure out a part number.

I like that idea. I saw that Summit and others make an inline residual valve. I really wish I hadn't chunked the old one so I could crack it open and see what they did to make it work, because I had decent brakes for a little while!

 

[BeerM3]

Well, so much for the notion of swapping out one from the older MC i had laying around. The "gasket" material on top of the little valve is unidentifiable but sticky. I'll have to check to see if rebuild kits come with them, but I think my best bet might be getting a properly sized MC with dual residual valves and archiving the one I have until it's time to put on the disc brake axle. Too many irons in the fire to start that little project right now!

 

[DSRTRDR]

that thing right there between the spring and the hex nut is the residual valve

why not use the entire old master cylinder then ? - caps should be same size, no ?

 

[BeerM3]

that thing right there between the spring and the hex nut is the residual valve

why not use the entire old master cylinder then ? - caps should be same size, no ?

That’s a fair question. It’s probably recondition worthy but has considerable rust inside the cups & back line brass insert/ residual valve. After talking with CruiserOutfitters, rebuild kits don’t typically include the residual valves, just the plunger, seals, and clips. If it was in better shape I’d probably be home free.

 

[DSRTRDR]

I may have a used master cylinder for you, but I won't be close to it for another two weeks - send me a reminder, and then I will search my parts pile in AZ . . .

 

[BeerM3]

That’s the residual valve from the front line. It took heat and a big ass cheater just to break that one free. The dissimilar metals had been too close for too long. I still haven’t budged the back one.

 

[BeerM3]

I may have a used master cylinder for you, but I won't be close to it for another two weeks - send me a reminder, and then I will search my parts pile in AZ . . .

Thank you but I already ordered a new one today from Bryce @ cruiser outfitters. This is my wife’s daily driver that’s been on jack stands far too long. She’s been very understanding but summer’s here and Joyota needs to be back on the road! I really appreciate the offer though!

 

[DSRTRDR]

CruiserOutfitters will set you up right - also think of them when you need carb rebuild parts or new spring bushings, shackles, spring pins and such