[Dave Burchfiel]

Doc, thanks for the input. I have the check valve and the brakes are properly bled etc. I've been doing a lot of research on this site and others.
and I've found I'm not the only one who has converted to vacuum brakes who's still having problems. The following is a post and reply from Hydratech. I've ordered a hydraboost unit (not cheap) and I'll let everyone know the results when it's installed. Probably be a few weeks. Even if it performs as advertised I'll be upgrading to 12" front and putting discs on the rear sometime in the future.

I have a Buick GN that has terrible brakes. I converted from the powermaster to vacuum brakes about 14 years ago. I'm running 16x8" wheels with 245/50 tires. I have three problems; the first was present with the powermaster and that is no matter what I can't lock the brakes. the 2nd and third are only with the vacuum conversion. The pedal travels a long way before the brakes even start to engage and on occasion, usually when the car is cold the brakes have no power assist at all and standing on the brakes with both feet barely stops the car from a slow roll.
I've changed the master cylinder three times and the booster twice. The vacuum canister doesn't leak, the lines are all good and have been replaced several times. I'm stumped. Could the combination valve be causing some of the problems. I guess the master cylinder has a "quick take up spool" (not sure what that is) but I think it stops the front brakes from applying before the rear drums have begun to engage. The master is from a 1987 Monte Carlo SS. The rest of the brakes are stock except for semi-metalic pads and soft compound shoes.

I'm pretty certain i'm going to convert to hydraboost. Will this take care of my problems? Do I need a different master cylinder? I'm eventually converting to Baer brakes. 13" front 12" rear. Are thera any special considerations for the hydraboost kit with that setup?

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Hydratech
Site Admin


Joined: 24 Jan 2005
Posts: 149
Location: MotorCity
Posted: Mon Jan 30, 2006 12:02 pm Post subject:

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Hi Dave - Yes, our GN systems will definitely knock your socks off with the power that they can produce! (guaranteed in writing or your money back in full). These systems are also totally consistent, with the same brake response at all times - whether it's cold or hot, whether you're running ten mph or 120 mph. Reliability is also second to none, which is why GM has decided to start installing these types of systems on most all of their truck lineup these days.

It sounds like your vac setup isn't running all that well due to the SFI Turbo V6 not particularly pulling enough vacuum, especially when cold. The factory G-body QTU (quick take up) design 3 piston master cylinders are pesky, as the valving inside usually doesn't work right - this could also be a factor in your situation. I am actually currently running a dead stock '86 442 (55k original miles), and have installed one of our hydraulic brake assist systems in this vehicle, (what a difference in braking with no other changes!), to find that I can't wait to get rid of the stock QTU design mc. Depending on just how fast and hard you hit the brakes, the pesky QTU design MC will just simply drive me nuts in use per inconsistent braking actions. I have swapped out the MC with a brand new AC Delco unit in thinking that maybe the original MC was tired, just to find that there was no improvement = I now recommend against the use of these types of master cylinders, as the idea seemed to be great "on paper", though just doesn't seem to really work properly in actual use. GM has also discontinued use of these types of MC's in many various applications (where they were used for years), apparently for the same reasons why I don't like these things. My experience tells me that your MC is most likely suffering an internal check valve problem, to where the QTU function isn't working correctly to cause your low pedal complaints. The idea behind these designs is this: When you first hit the brakes, the high volume oversized rearmost piston is *supposed* to take up any slack in the system first, then a check valve kicks in to shut off the QTU function and then follow through with the forward two conventional smaller MC pistons to produce the pressures the brakes need. As you can imagine, this action is subject to various little metering / check valves inside of this type of MC working properly, which can malfunction with even the slightest little piece of dirt or wear. If the internal valves aren't working correctly, you can experience so many different problems that it would even be quite a challenge to list all the possible symptoms. Again, this is exactly why I don't like these designs, and generally recommend against their use...

QUOTE: I'm pretty certain i'm going to convert to hydraboost. Will this take care of my problems?

REPLY: YES - satisfaction guaranteed of your money back in full.

QUOTE: Do I need a different master cylinder?

REPLY: Yes - the QTU design oversized rear register of the '87 Monte SS MC (the rear part that actually fits into the booster) will not fit the "mouth" of the replacement hydraulic assist unit, so a conventional 2 piston design mc will be needed. We have bored the assist units open larger to fit the factory G body QTU MC's in testing, (like my 442), to find that even though we *can* make them fit, that we simply just out and out do not recommend the use of this type of MC. To that end, there are quite a few various factory type of MC's and a host of aftermarket MC's that you could consider installing (as the assist unit is designed to accept most all '77 and newer design GM non-QTU power brake master cylinders - you could even use a '96-'02 Z-28 or '97-'02 C5 Corvette plastic / aluminum if you wanted to). The target bore size should be 1 1/8", though the 1" bore versions will work OK too.

QUOTE: I'm eventually converting to Baer brakes. 13" front 12" rear. Are thera any special considerations for the hydraboost kit with that setup?

REPLY: Even though the booster is hydraulic, all of the usual conventional wisdoms still apply from the MC forward. To that end, there really isn't anything you would particularly have to do differently because of having a hydraulic brake assist unit (as opposed to a vac booster).
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