What master cylinder/proportioning valve to use?

[MDPotter]

I picked up a hydroboost from the junkyard this weekend. The junkyard didn't have any first gen Astro vans so I ended up with a unit off a 2008 Duramax. I'm converting from manual drum/drum to hydroboost with front discs, but what master cylinder and proportioning valve do I need? Can I reuse my stock master cylinder? I'm assuming the drum/drum proportioning valve isn't right for a disc/drum setup. I measured the bolt spacing on the hydroboost to my stock master cylinder and they appear to be the same.

This is in my 68 GMC C15 2WD.

Pics of my stock setup and the hydroboost:

[Rickysnickers]

Yes you should be able to reuse your existing M/C. You will need a new prop valve for a disc/drum set up. or you could get an adjustable valve. If you ever decide to put rear discs on your truck, then you are ahead of the game with the adjustable unit.

[MDPotter]

I'm fine with a disc/drum valve, can you give me some direction on part # or year/make/model?

[jeffahart]

I would get the master that originally came on the booster. You can use Inline Tube prop valve # PR102. I am not sure about that silver accumulator. I know for sure the gold one is for disc/drum. You need a disc/drum accumulator for an original GM disc/drum setup. That silver one may be a disc/disc.


j

[MDPotter]

Originally Posted by jeffahart

I would get the master that originally came on the booster. You can use Inline Tube prop valve # PR102. I am not sure about that silver accumulator. I know for sure the gold one is for disc/drum. You need a disc/drum accumulator for an original GM disc/drum setup. That silver one may be a disc/disc.


j

I found a proportioning valve from Speedway that should attach to my stock M/C: https://www.speedwaymotors.com/GM-Di...lets,9878.html

I'm sure the 2008 Sierra I pulled the Hydroboost off has 4-wheel discs. Even with the stock M/C and a disc/drum valve, will that be an issue?

[jeffahart]

You will be needing a Drum Residual Pressure valve for the rear. The disc/disc accumulators are charged at higher pressure. You may be "ok" with it, but why chase your tail. It might be better to get setup with compatible components. Then as you swap stuff out later, you'll know what's wrong. If you can start out with a solid working baseline then you can swap out for performance or looks or whatever later. Anyhow, I think the disc/disc accumulators are charged to 450psi and the disc/drum 350psi. Will it work, yes... but I really doubt you will get optimal performance for your efforts.


j

[bs46488]

I've done 2 hydroboost swaps like this. One with the astro booster and one with the silverado booster.

I swapped master cylinders around a lot trying to get the pedal to feel good.

All the internet buzz I read said you need a MC with a 1.125 bore

In the end I ended up using the factory MC....which had a 1.125 bore.

I would get the booster for a 1968 with disc brakes and attach it to that hydrobooster.

Plus I think it looks best in our trucks.

[67 chevelle]

Originally Posted by Rickysnickers

Yes you should be able to reuse your existing M/C. You will need a new prop valve for a disc/drum set up. or you could get an adjustable valve. If you ever decide to put rear discs on your truck, then you are ahead of the game with the adjustable unit.

Agree ,, Ive done several hydroboost conversions ,the stock master will work well , you need a disc/drum combo valve if that is whats on the truck .

[MDPotter]

Thanks for the info everyone. I think I'll stick with the hydroboost, reuse my stock master, go with Speedway's disc/drum valve, and put in a 10 psi residual.

[Rickysnickers]

I don't know as you will actually need a residual pressure valve for the drum brakes. My understanding is the only time you need one is when the m/c is below the level of the drum brakes, such as when the m/c is mounted on the frame. Before buying one, you may want to look into whether you really need it or not.

[jeffahart]

I mentioned the RPV so he would be aware it's needed. The combination valve he's looking at should be good. As a side note: you can pull the RPV from the front-brake side of a drum/drum master when adding disc up front(pull the brass fitting, remove the spring and rubber check valve, reinsert the brass fitting). You can stay with the original distribution block. Only problem is, you have to monitor the small reservoir as the front pads wear. Bottom line, pressure for the rear drums need to be taken into account. I am running the prop valve I posted and didn't even look to see what was in the master, it was off a 1986 CUCV disc/drum so I left it be and ran it. System is good, but I used everything for a factory setup for a one ton truck... except I'm using that after market combination valve. I would try to setup without an adjustable at first. I think you should try to get close on the first pass.

So the big question: does the combination valve hold the 10psi for the rear drum - OR - does it only manage pressure to the rear during hard braking.

Just know this going into the upgrade, then you can better diagnose your pedal behavior.

Here's what you get in the combination valve: Metering to hold off pressure on the front allowing the rear to apply first. Differential valve for safety, it's what the wire is hooked to for the idiot light. Proportioning so the rear doesn't skid during hard braking(and only alters pressure to the rear during hard braking).

Match components that are designed to work together... give yourself a brake! (pun pun funny.. right?)




j

[4carbcorvair]

From my understanding, the residual valve holds 10psi to the rear drums as the return springs on the rear shoes will overcome the pressure not allowing the drums to work effectively. Which is what my issue with my Jeep CJ is currently. I'm running a dual diaphragm booster and a Corvette 1.18 disc/disc master. The problem I'm having is the rears barely work. In snow, I have to really step on the brakes to get the rears to lock, in the meantime, the fronts have been locked and sliding. From my understanding, the residual valve for the drums will maintain light pressure (10psi) so when the brakes are applied, the shoes push out and work as intended. I was looking at installing an inline residual, they are only about $15. With the Vette master, I believe it is a bit too large for my stock brakes also, as the pedal is very firm. Instead guessing/trying, I'm going with a 95 Jeep YJ booster and master for disc/drum. Just arrived today and hoping to install later this week.

[burnin oil]

For the guys looking for the brake parts. Go to Summit racing. They carry about every part that you could need and usually have about the best price. I bought a disc/drum proportioning valve there for a 70's chevy in the past. There really isnt a whole lot to them so it should work out just fine. Fittings and wiring connections are really the big differences. Heck it went on a 90s Dodge 3/4 ton. They also carry a mess of valves for the drums. I would assemble without it and see what happens. It is simple enough to add later. Some setups seem to work better without them and trial and error is the only sure way to know.

Hydroboost units are pretty generic with the major differences being the mounting plate, connector rod for the pedal, and fitting size. The internals are about identical. When my dad rebuilt his the company he got the kit from only stocked two different ones. This was a large rebuilder and not a specialty outfit. The kits where just like any major overhaul setup that contained an extra seal or two but it just shows how universal they really are.

[jocko]

While an original drum/drum mc will work for your scenario, it is not necessarily the correct approach, even with an adjustable disk/drum prop valve. There is a difference between drum/drum and disk/drum mc’s. It’s worth spending the $25-30-ish bucks for a correctly matched system. It’s not the fact that you’re converting to hydroboost - it’s because you’re converting from drum/drum that drives the need for a correct mc. (In other words, if you were converting a 71 or newer to hydroboost, no you would not necessarily need a new mc).

[MDPotter]

In comparing AC Delco master cylinders from a 68 (drum/drum) to a 72 (disc/drum) on Rock Auto, they are different part numbers. The bore is different too - 1" on the drum/drum vs 1.125" on the disc/drum and the reservoir is taller. It's a $40 part so that's not too bad.

[4carbcorvair]

Originally Posted by jocko

While an original drum/drum mc will work for your scenario, it is not necessarily the correct approach, even with an adjustable disk/drum prop valve. There is a difference between drum/drum and disk/drum mc’s. It’s worth spending the $25-30-ish bucks for a correctly matched system. It’s not the fact that you’re converting to hydroboost - it’s because you’re converting from drum/drum that drives the need for a correct mc. (In other words, if you were converting a 71 or newer to hydroboost, no you would not necessarily need a new mc).

Agreed.

Do it correctly, it's worth it. I just swapped out the booster and master on my Jeep CJ yesterday. Stock front disc/drum. I had a Corvette maser, 1.18 for disc/disc. Disc/drum prop valve. Firm pedal and the rear were pretty much useless. I swapped in a dual diaphragm booster and master from a 1995 YJ with 6cyl which is a 1 inch bore and set up for disc/drum. I can no lock all 4 up without issue. Perhaps the best the brakes have worked even when new.

In short, brake systems are set up to work together. Get the correct parts.

[MDPotter]

My plan is to use this master cylinder: https://www.rockauto.com/en/moreinfo...496333&jsn=644

And this proportioning valve: https://www.speedwaymotors.com/GM-Di...Query=91031351

And this residual pressure valve: https://www.speedwaymotors.com/Wilwo...ry=83526013784

Will the hydroboost I pulled off the 2008 duramax (disc/disc) work with these parts and my disc/drum setup?

[jeffahart]

Originally Posted by jocko

While an original drum/drum mc will work for your scenario, it is not necessarily the correct approach, even with an adjustable disk/drum prop valve. There is a difference between drum/drum and disk/drum mc’s. It’s worth spending the $25-30-ish bucks for a correctly matched system. It’s not the fact that you’re converting to hydroboost - it’s because you’re converting from drum/drum that drives the need for a correct mc. (In other words, if you were converting a 71 or newer to hydroboost, no you would not necessarily need a new mc).

For sure not correct. I did have trouble when I moved to front disc on this truck with the newer master and prop.I ran the drum/drum with yanked RPV for a few months. I finally got around to installing the correct master setup. I was dissatisfied, and it left me scratching my head for weeks. It drug its hind end and I hated the feel of the pedal. I went back to the original drum/drum distribution block and master with the yanked RPV. The brakes worked great... go figure. I only have so much brake bleeding in me. I got sick of dickering with the brakes. And I refused to put an adjustable on the rear, call me stubborn. When I upgraded the booster I decided to give the Hydro a whirl. It seemed like a cheap easy upgrade... it was. I am delighted.

If you read any of my posts, you will always see me advocating for a working baseline. Once you start mixing and matching components you can really start chasing our tail.

Will the hydroboost I pulled off the 2008 duramax work (disc/disc)

I think you should get a booster with a disc/drum accumulator. As long as you're at the dismantler get the master with the booster, get them attached, no mistakes. You will get the correct rod from the booster to the master. Further, if you get a junkyard booster only, make sure it has the rod and if you ever take it to a parts store for core make sure you pull the rod first. I usually, take stuff back for core after I get the reman installed and working. Maybe someone here can advise on the behavior of the disc/disc accumulator on your setup. My thought is that these are newer components and are plenty available. You can get real close or even slam dunk on your first pass. And have brakes that put your face in the windshield... that's what you want.


j

[MDPotter]

Originally Posted by jeffahart

For sure not correct. I did have trouble when I moved to front disc on this truck with the newer master and prop.I ran the drum/drum with yanked RPV for a few months. I finally got around to installing the correct master setup. I was dissatisfied, and it left me scratching my head for weeks. It drug its hind end and I hated the feel of the pedal. I went back to the original drum/drum distribution block and master with the yanked RPV. The brakes worked great... go figure. I only have so much brake bleeding in me. I got sick of dickering with the brakes. And I refused to put an adjustable on the rear, call me stubborn. When I upgraded the booster I decided to give the Hydro a whirl. It seemed like a cheap easy upgrade... it was. I am delighted.

If you read any of my posts, you will always see me advocating for a working baseline. Once you start mixing and matching components you can really start chasing our tail.



I think you should get a booster with a disc/drum accumulator. As long as you're at the dismantler get the master with the booster, get them attached, no mistakes. You will get the correct rod from the booster to the master. Further, if you get a junkyard booster only, make sure it has the rod and if you ever take it to a parts store for core make sure you pull the rod first. I usually, take stuff back for core after I get the reman installed and working. Maybe someone here can advise on the behavior of the disc/disc accumulator on your setup. My thought is that these are newer components and are plenty available. You can get real close or even slam dunk on your first pass. And have brakes that put your face in the windshield... that's what you want.


j

The Duramax disc/disc booster isn't what I really wanted, but I didn't know at the time that it could be an issue for being disc/disc. What I really want is a hydro with master cylinder and proportioning valve off a first gen Safari or Astro (85-94), but they only had 2nd gens (95-05) which have a severe downward angle where they attach to the firewall that may not work.

What hydroboost unit are you running? And are you saying to install a RPV right away or not? I'm fine with trying the stock drum/drum M/C, getting a new disc/drum prop valve and seeing how the brakes behave.

[jeffahart]

Way above there somewhere I said I didn't ever bother looking at my master because it came off a disc/drum(I assume it has an RPV there for rear drum, but I didn't look) I just trusted the setup because it came off a K3500 disc/drum. Oh, and you can rest assured if I have to replace that master.. I'll for sure look at it good to verify I get and exact match from the parts house. And that means any RPVs that might be in there now.

I think you could work the phones or ebay and get the setup you want. Mine came off a 85 CUCV and I did it on purpose and I'll write a note on that below.

So another point: Knowing what you have. Believe it or not, you can start to forget what the hell you put on there a few years down the road... Seriously! I put on an axle set off an 85 CUCV. When I went hydoboost it was a simple choice for me; I got everything, including brake lines, for an 85 k3500 only after market item is that prop valve. Anything I need from the pedal rod to the end of the hub I can just ask for off an 1985 k3500 diesel. Easy right? I like the easy button. But like I said, the disc/disc accumulator may be fine. I guess you could just end up with a touchy pedal.

Remember, your pedal and mounting bracket is going to be based on your hydrobooster, so do your due diligence now. There is much writing on it. And many parts combos that "could" be successful. But once you pull the pedal assembly and all that crap, you ain't gonna want to do it again to reconfigure for a booster that mounts differently.


j

[MDPotter]

I think what I'm going to do is go back to the junkyard and return the booster I bought and pull one from a 2nd gen Astro, complete with the master cylinder and prop valve. I have determined that my stock cylinder needs the long pushrod which is not what the 2008 booster is. Here is a link to Hydratech's website - at the bottom they have pictures showing the differences in the pushrods. I wasn't aware of these differences and I agree that it makes sense to get a complete unit. I had read in another thread that maybe the 2nd gen Astro units wouldn't work because of the downward angle, but I've since found a thread by Captainfab for his hydroboost brackets and plates and it looks like several people have used the 2nd gen setup without issue.

http://www.hydratechbraking.com/GM_67-72_CK.html

[jocko]

Originally Posted by jeffahart

For sure not correct. I did have trouble when I moved to front disc on this truck with the newer master and prop.I ran the drum/drum with yanked RPV for a few months. I finally got around to installing the correct master setup. I was dissatisfied, and it left me scratching my head for weeks. It drug its hind end and I hated the feel of the pedal. I went back to the original drum/drum distribution block and master with the yanked RPV. The brakes worked great... go figure. I only have so much brake bleeding in me. I got sick of dickering with the brakes. And I refused to put an adjustable on the rear, call me stubborn. When I upgraded the booster I decided to give the Hydro a whirl. It seemed like a cheap easy upgrade... it was. I am delighted.

If you read any of my posts, you will always see me advocating for a working baseline. Once you start mixing and matching components you can really start chasing our tail.

Jeff, as I mentioned in my first sentence - using a drum/drum mc on a disk/drum mod'ed vehicle will indeed work. The point is that the internal fluid passages of a disk/drum mc and a drum/drum mc are different for a reason. The factory distro block on drum/drum systems was only that - distribution and a place to mount the warning switch for the brake circuit isolation valve - any front-to-back biasing was done internally in the mc circuitry, not the distrib block. Anyway, the delta of using a drum/drum mc in a disk/drum setup can certainly be adjusted out and tinkered with via an adjustable proportioning valve until it feel about right - or it may work just fine as your example highlights, but that doesn't mean it's optimized. The drum/drum mc was still accomplishing its internal biasing - yet you had a disk/drum setup, so it wasn't the factory designed biasing for a disk drum system - even though it still function fine overall. That's really my only point in this discussion.

I agree completely with having a baseline from which to deviate, but in my experience, the best working baseline has been the factory setup - that's why I advocated starting there for the OP, especially if it's only $20-30 to have a brand new disk/drum master cylinder, and then deviating to one's taste/preference to modify the system.

A dragging rear brake system when the only change made was the installation of a disk/drum mc doesn't mean that using a disk/drum mc on a disk/drum system is a bad idea (the factory did it on every truck they produced from 71 forward)- it means there was some problem within the system, perhaps even a defective new mc. Anyway, just wanted to clarify. There is a reason GM didn't just continue using the same mc from pre-71 when they changed to a disk/drum system in 71.

[jeffahart]

Originally Posted by jocko

I advocated starting there for the OP, especially if it's only $20-30 to have a brand new disk/drum master cylinder, and then deviating to one's taste/preference to modify the system.

Yes, totally agree. What I was saying or thinking I was saying. I probably just mucked up the point. I installed a almost complete system off an 85 one ton. I really like that it worked after assembly and no dickering around.


j

[jeffahart]

Originally Posted by jocko

A dragging rear brake system when the only change made was the installation of a disk/drum mc doesn't mean that using a disk/drum mc on a disk/drum system is a bad idea

Of course it's not a bad idea to use the proper MC. Never said that. But... it was a good idea for me. Not advocating it. Just a fact and I was the one working through it. I got sick of messing with it and stepped back to what worked. When I moved forward again it was with a system off the one ton.


j

[MDPotter]

Thank you everyone for the feedback, I'm learning a lot here.

The way I see it, I'm faced with two options now:

1-Keep the duramax disc/disc booster or pull a booster off a 2nd gen Astro (downward angle requires new master when modifying mounting plate to eliminate angle). Disc/drum prop valve from Speedway. Find a master cylinder that is made for the short pushrod which both of these boosters have-perhaps a master cylinder for a 96 3/4 ton diesel (disc/drum factory)?

2-Search for a 1st gen Astro complete setup (booster, master cylinder, prop valve).