1972 Chevy MH FC 'Atlas 21' Braking System Questions

[Axle]

Hello Everyone-I am working on a 1972 Atlas 21 with a Chevy motorhome/forward control chassis. Many of the parts cross to a P30.The brake system is something else. The master cylinder is manual and drives two remote slave cylinders with their own vacuum boosters. Front is disk, rear is dually with drums.It's been mostly a driveway decoration for a few years, only about 300 RV'ing miles over three trips, and the MC always seemed to loose fluid. Not thinking hard, I would just top it off before any driving. I went to check it this year and the filler cap was off, fluid gone, rust everywhere.

I pulled the MC thinking that something Chevy should not be hard to find...but this combination: 1.25 bore, manual, oval lid, 2 bails, and 9/16 and 1/2 ports on LH side is somewhat of a unicorn. So far I have been sold one with everything right (bore, casting, lid, ports) except not the manual piston hole (not re-workable, not enough metal) and one with the right hole but not the right sized ports. I pulled the old MC apart and seals are toast and casting of primary piston is also almost gone at the rearmost seal.

NAPA found just a replacement piston for 100+$ or I could try 'frankensteining' the two 40$ rebuilds together or send the current on in for a 250+$ rebuild. The chassis build label got water damaged  so I do not have brake codes and crossing the current casting got me the above results. There is no assurance that what I had on there was original to the chassis but previous owners did not seem to be proactive on this sort of stuff.

Any ideas for a replacement MC or people to talk to or words of experience or encouragement?
Thanks and happy RV'ing!

[Preese]

Hey Axle,
Had similar problem with ports on new MC not matching brake line fittings.  Luckily O'Reilly's guy was willing to dig around in back and find me a bushing that made it work.  Also NAPA inventory seems to vary between stores.  Happen to stop In a different NAPA and they had a display of brake line fittings that my local NAPA did not carry.  Best of luck.

[Axle]

Thanks Preese. An adaptor/bushing would do the job for changing the port size.

I am a little concerned that the replacement cylinder will not output fluid the same as the original, although anything is better than nothing at this point. That is why I am being insistent on the bore being 1.25. I am going to try taking apart the second cylinder and seeing if a part swap between pistons is possible. If I can do that and return one as a core it's not so bad money-wise. I may have another core to contribute if the store is willing. I will keep the 'original' until everything is up and running.

[Rickf1985]

You are talking about your brakes here, part swapping between good and bad master cylinders is NOT a good idea! The specs on the new one should give the piston size and if it is the same as the old one then that will be what you need. Hydraulics work on a piston to slave size ratio and as long as you match that ratio you will be good. Smaller piston will give more pressure to the slave (wheel cylinders) but will also give you more pedal travel. Larger piston will give less pedal travel but less pressure at the wheel cylinders. Fittings can be overcome with adapters and even if you have to change sides if you have room to do so you could run new lines.

[Axle]

Thanks Rick.To be clear, I was thinking of switching the manual pushrod piston from the rebuilt #2 cylinder that has the right piston but a different casting and different port sizes and putting it into the rebuilt #1 cylinder that has the correct casting and ports but currently has a power brake piston (with the divot not the hole as I need). All this is conditional based on if they match in size of stroke, bore, etc... and are a reasonable match to the original piston for reference. I was not going to put the old seals and old piston into a fresh rebuilt master cylinder.  I thought about using new seals and old piston in the rebuilt #1 cylinder but the old piston has damage near the rear seal groove.
Here is a couple pics of the MC's next to each other. It looks like the one I call #2 came with adaptors but there is not the correct one to make the 'green' port into a 'red' port. If I solved for that I could use #2 without a piston swap and not spend the money on the other cylinder. The #1 seems to have 1 plug that is not right either. Hmmmm...

[Axle]

Delusions of Grandeur...
I tried the primary piston swap, moving the re-manufactured manual push rod piston into the re-man casting that had the correct fittings and shape as the previous.
I was not able to get any bleeding or pumping action out of the secondary piston on the bench while the primary would squirt DOT3 halfway across the room if it were not for myself acting as a cloth-covered shield for the rest of the garage. I could see piston movement through the holes in the reservoir but that was it.

So I am un-swapping the pistons and thinking about using the casting and internals intended for the manual push rod piston but this will mean I need to find an adapter as one port is one size smaller than needed. Or I send the one that came off in for a comprehensive 250$+ rebuild which might involve a shiny new 100$ piston (as quoted as a separate piece by the rebuild shop, a well known local firm used by the regular parts houses when brake questions get too deep) as the previous one is missing metal close to the rearmost seal.

So much for 35$ and a 7$ core charge...

[Oz]

Well, you'd never known if you hadn't tried. 
Onward from here.

[Froggy1936]

TIP: On squirting fluid all over the place,, Make up short sections of line that go from ea port back to the reseviour with the lid off As you bench bleed the cly the fluid will stay near full and no mess . Frank

[Axle]

All right, I am back on this job again...
I got a MC in from Pirate Jack. Everything was looking good as I was ordering but the ports were in the wrong order on the cylinder. Talked it over with them and decided to try their cylinder with adaptors rather than return it. I also got the bleeder kit with the barbs and some clear hose to watch the bubbles and fluid proceed safely back to the reservoir as suggested above.

I started to bench bleed. Pumped 40-50 times, still some bubbles. Then I let is sit over dinner, came back, not so many bubbles. I will cycle a few more times and attempt to install.

[Oz]

Hope it works!

Actually, I could have saved you a lot of grief here.  If you ever have a brake parts question,  call Geoff at Alretta.
He KNOWS these old brake systems better than anyone.  Been doing it for over 30 years and getting our members the RIGHT parts, the FIRST time!

www.alretta.com

[Axle]

No worries. I stumped several parts houses and our regional rebuilder of brake oddities too.
If the current one ends up failing soon I will look into that vendor.
I am about half done, ended up sanding and primer-ing as far as I could reach. Lipstick on a pig but why not let the pig feel special for a minute...
I tried sticking the gasket to the cylinder with a little double-sided tape but it stuck to the firewall better.
My 20-something daughter helped. By then end of the session she was telling me to move over so she could reach the difficult parts. First time that has ever happened, makes me proud of her abilities. I really need to get a 9/16" offset wrench for this job.
Some pics enclosed. I still need to  hook up the brake lines and  re-install the old pushrod with the new boot.  I dropped the pushrod off the pedal to have one less moving part to align wile mounting the cylinder. There is poor access to the MC area, I was temped to add a access hatch there but I do not need another project...

[Axle]

Done for today. Lines are hooked up and pushrod is back in and system is ready to be bled.

[Oz]

I bet your ARE proud of her!
Good job.
People made an access door to the MC, including me. Lol!