I converted my Dart to a hydroboost setup last year I think (time flies). In that conversion I wasn't able to find a suitable dodge hydroboost and ended up with the unit from a 94 Astrovan (GM stuff). The problem is the MC that came with them have a tall plastic reservoir that wouldn't clear and a 1.25" bore. So I searched around to find one that was shorter but would have the right piston depths and specs to fit the hydroboost unit. I think what I ended up with was a 78-80 Corvette Power assisted MC. It works, but the piston diameter on this GM stuff is all 1.25". When I have no hydroboost (parked, engine off) I have almost no braking ability at all. I'd like to improve the boosted braking AND have some better potential for manual braking in case of emergency. Where can I get a 15/16 to about 1" MC that will fit (right pushrod depth and imperial brake line fittings and no dorky tall plastic reservoir.

Existing unit has these markings on it:
5460346 or 5460946 hard to read these cast numbers
Hy: 286 I think this is a # of the hydro unit

Best pic I have at hand:
http://www.telusplanet.net/public/zbhansen/temp/HydroboostMasterCylinder.jpg

Thanks for any help in getting a better MC selection.

Can't help you on the master pn but an observation... with the big assist from the hydroboost plus a small master diameter, you will be able to generate crazy high brake line pressures. It could be too much for the lines and wheel cylinders/capilers. Suggest you contact Hydroboost and ask minimum recommended master diameter.

For the brakes calipers I have, originally the mustang hydroboost units used a 15/16 master. There is a difference in the size of the Hydro unit the mustang used, but it's not a great % if I loosely recall the specs. The mustang brake system seems to be designed around lower fluid volumes (smaller surface areas of MC and caliper pistons) and higher line pressures. So I'm hopeful that I'm not really exceeding anything that calipers cannot handle already. Yes, I do not want to blow seals or have an unsafe over pressure condition either. I'm not even 100% sure the MC I have is 1.25" but the astrovan unit for sure was. The switch fom 1.25" to a 1" is about a 57% increase in possible line pressure. That said, before I had the hydro I had a vacuum assist with 15/16, 1-1/32" and 1-1/16" Master cylinders and I would call those borderline unsafe which is why I went to the hassel of hydroboosting it.

In general driving maximum line pressure is never reached. In emergency braking When teh wheels stop spinning the foot comes off the brakes naturally as that happens so not likely to exceed any limits and certainly not sustain them. I feel safer operating with that limit than not having enough in a more likely scenario where I lose engine power for any number of reasons, my lousy home brew electronics being one very likey reason.

maybe you can find a way to put a hydraulic accumulator into the system somehow, allowing you to get 1 or 2 pumps of the brakes if you do lose engine power.

I don't know if they make such a thing for any of the GM hydroboost set-ups, but aircraft use them a lot.

Hydroboost units have a built-in accumulator. It's good for about 2 pumps if it's fresh depends how much you module the pedal as you use it.

I did more digging on that part number and it's just confused me a bit more. It may already be 1-1/8". It may be from the 67-69 camaro time range, or it may be from 77-82 corvettes. GM seems to re-use that part number somehow. Unless any certain part options pop up I'll just measure the specs on the neck, piston and pushrod seat and make sure I find something compatible from spec from some vendor or NAPA listing.

Try the 86 thru 90 caprice wagons. They were all 307 olds w/hydroboost, but they may have had a tall master too.

Any updates on this? I too have just got an astro van H/B and am going to put it on my Challenger this winter. Cass at Dr Diff suggested a 1 1/8 master with the H/B so I'll get one off him to try.It's the lines that have me wondering at the moment? What did you do for lines? What pump are you going to use? I thought I'd just use the stock power steering pump on the car?

That looks like a standard Corvette type master cylinder. They are available in 15/16", 1" and 1 1/8" bore.

I don't recommend going smaller than 1 1/8" due to the high line pressure the hydroboost can create.

Love the hydroboost in my '93Sub. I bought a new master cylinder for it on the 'bay for $15. It's aluminum but probably too tall in the reservoir.
After trying to work around the vacuum power brake unit on my $500 ford pickup I would think hydroboost wouold be preferable on about any car, clearing up underhood space.

R.

Call Vanco, he is the master at hydroboost! They use them all of the time in Jeeps and Hotrods, he is the resident expert on Hydroboost! Tim

http://www.vancopbs.com/category_s/66.htm