Hydroboost question

[yossarian19]

I'm running a '98 Astro HB unit and it is soon going to be time to replace my master. I don't like the current unit (it's a Astro MC with S10 reservoir) and want to swap to a C10 iron model without the plastic reservoir - what I'd call the "old as heck lookin' " type.
My question is if the old C10 master cylinder will bolt onto the Astro HB, or do I need to find something else?

[ls1man]

I used a 77-81 corvette, $35 on ebay.

[yossarian19]

That's the one people are running for disc / disc setups; I'm running disc / drum, though.
I will probably pick up a C10 master & compare it to the S10 master on my shelf, or maybe just put the S10 master in (wrong bore size but hey, it's going on a '57. Any and everything is an improvement over the '57 parts)

[1project2many]

I can't tell you if a specific vacuum m/cyl will work. I can tell you what to check when swapping m/cyls. Many of the aluminum units for vacuum won't work because the OD of the aligning sleeve on the rear flange is too large. The iron units seem to have a smaller sleeve, even smaller than the HB units. I can't say what negative effects this may have... probably no practical issues to worry about.

You also need to watch the depth of the hole in the piston. They vary depending on which m/cyl you choose.

Here are some photos to study.

[OrrieG]

I know nothing about hydro boost, but thinking out loud here. I know the volume and piston movement of the MC have to be matched to the rest of the braking system. The Astor van MC looks much smaller than the C10, I don't know if having more fluid and larger bore will affect the braking by allowing the calipers and drums to be energized before the MC piston can clear any internal ports??? Or result in too "touchy" of a pedal because of shorter required travel???

[yossarian19]

A larger MC will reduce system pressure but engage the brakes more quickly. To compensate, Chevy used HB units, different pedal ratios and on vacuum systems I imagine they used larger diameter boosters to compensate for the larger MC. The larger bore cylinders are nice in that they engage the brakes in the first little bit of pedal travel, wheras a smaller MC would have to be 2" down in the pedal before it moved enough fluid for a 3/4T caliper to engage the rotor.
There's a bit of leeway in all this, though. I looked at the factory caliper / cylinder bores & MC bore pairings for a ton of different vehicles and tried to match the OEM Astro system as close as I could. There's no law saying you have to do it that way .
It looks like the 74-91 HB unit might be the one for me. I'm not positive that the engagement to the booster is going to be right but if I bring the Astro MC from the truck & a few measuring tools to the parts store, I'll come up with something.
I appreciate the help, guys.

[1project2many]

There's a bit of leeway in all this, though. I looked at the factory caliper / cylinder bores & MC bore pairings for a ton of different vehicles and tried to match the OEM Astro system as close as I could. There's no law saying you have to do it that way .

That's a helluva lot better than many do. When I converted the S10 to HB, the catalog info I found showed all HB m/cyls to be the same diameter up to the 2000 or so Astro. And they were same bore front and rear. Apparently the Ford Mustang units come in two different diameters, "early" and "late" and that makes me wonder if there are more variations. Catalog info is getting harder to find online but if you need it I might be able to dig some up.

[spreader]

Is this what you are looking for? The HB is from a 3/4 ton mid 80"s Chev/GMC. They all look the same to me as to physical size. The MC # I can't remember but I can get you the number fairly easy. I used the same HB on my scout II, but a Corvette MC ca. 68-72. Good for disc/drum or disc/disc. Main thing to watch out for is the rod into the MC from HB(1&1/8" vs 1/4" depth) and the rod from the brake pedal to the HB. You have to have about 1/16"-1/8" free play there.