Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Redundant Brake Boosters Safe? (advanced mechanics)

Im replacing the master cyllinder on my 1978 Chevy C60 2-ton truck with one from that years’ Silverado. It will retrofit fine but only on the condition that I leave the Booster mounted to it. The concern I have is that this truck already has another huge, non-integrated booster located in the undercarriage. The only extra step I have to take to connect the additional one is to add a T-connector so both hose lines can share the vacuum off the carburetor.

My main question is would their be anything unsafe about this setup?

Also, they would probably share pressure in the brake line so one booster would do the majority of the work and the secondary would hardly do anything. Correct?

Could you confirm that it would provide an additional measure of safety, being that if one booster failed the other would come in?

Background: Im doing this primarily to avoid both paying an arm-and-leg for original replacement, and also the fact that the original replacement has the tiniest fluid reservoir for a 2-ton truck versus a substantial one off the Silverado. Also adding it would minimize future replacement costs and possibly add additional safety.

You seem to lack even a basic understanding of that brake systems operation. Leave the brake work to someone who knows what they are doing or close the hood and do some serious studying of hydraulics in general and brakes in particular.

So far, you havent shown that you do either. If you cant try to answer the question then dont post anything.

Excuse me.

"Could you confirm that it would provide an additional measure of safety, being that if one booster failed the other would come in? "

That’s not even close to how it could work. Do the repair by the book. If you’re going to redesign (modify) the braking system, you need someone to do it who knows more than you seem to know about it.

Upon reflection, I’ve decided that answer was a little harsh. Here’s some info for you.
If you connect vacuum to both boosters, then they would share the load to some extent. If both have diaphragms of equal surface area, then they should share equally. If not, then the bigger one would get more of the load. Of course, if you connect both, then that might take more vacuum than the engine wants to spare.
In practice, if one were to fail, then the most common failures would result in a large vacuum leak. This would likely deprive the other booster of vacuum such that it wouldn’t function after the first brake application either.
My biggest concern would be getting the linkage between the two boosters right. Any mistakes and you may suddenly have no brakes when you needed them most.
Perhaps you need to go further and change out the pedal assembly and all linkages to make it completely a Silverado in respect to brakes?

As succinctly as possible let me fill you in on the basics, C60, The master cylinder that is operated by the brake pedal is merely a linkage. It was preferable for the manufacturer to put the brake master cylinder and the booster under the cab. It was simpler to use a generic master cylinder as is used on a hydraulic clutch to actuate the brake system than build a Rube Goldberg system of levers and linkages…

Im more than happy to take the extra time to disconnect large factory booster and just run the system like a Silverado as previously suggested. My only reluctance is the belief that there must have been a purpose in having the giant booster system for a 2-ton truck. If it turns out that the large 1978 booster is antiquated and of equal or lesser effectiveness compared to the newer Silverado ones then yes, I’ll go ahead and run it exclusively on the Silverado.

Also, after checking out the workings of the current system, I saw nothing other than the vacuum hose that needs to be messed with on a dual-system configuration (add T-connector).

Im now just getting around to having the time to start work on this system in the next week or so.

Thanks for any continued replies.

I hope your insurance is paid and the roads are clear when you road test it.

I would strongly recommend against having two vacuum boosters. You gain nothing, but you could loose all braking if either one develops a leak. As I see it, nothing to gain and twice as many opportunities for a failure.

I would also be concerned that Silverado’s master cylinder might not provide enough brake fluid to move all the brakes. The truck may have larger capacity calipers and wheel cylinders than a Silverado and the new master cylinder may have smaller orifices that supply fluid too slowly.

Thats just it Keith, the factory master cylinder is this ridiculously tiny thing that looks like it was made for a Ford Fiesta! The one for the Silverado appears perhaps three times the capacity.

As far as the argument about leaks, well I dont claim to have particular knowledge and experience in the area of dual-brake boosters, but it defies common sense that a leak in a single or dual system would have any different effect. A vacuum leak should just mean it would be hard to brake but the hydraulics would still work.

Well folks, if I crash and total the truck, I guess we’ll have more definitive answers on this topic!

Thanks again all.

The appearance of the original master cylinder is the most superficial form of assessment.

I can’t tell you if your proposed arrangement is safe.

I suggest that you can save money with air conditioning repairs. If it doesn’t work, it’ll be warm. You can save money with engine repairs. If it doesn’t work, you can get a tow. If your inexpensive cooling system repair fails, you can turn off the engine to stop an overheating condition. In these cases, no one dies.

Rigging repairs for brakes or steering can be a fatal mistake. This isn’t a time to save money or worry about what the correct parts look like. This is a time to use parts the manufacturer specified and that have worked for the past 30+ years.

“Well folks, if I crash and total the truck, I guess we’ll have more definitive answers on this topic!”

How many lives is this answer worth?

Based on this rather rude response, I think Rod’s original answer should be modified:

“Yes, absolutely it will work in that doing what you propose will indeed do something. Since we are unaware of anyone else trying an idea like this, we can only go on theory, but in theory it will probably do something that you won’t like. But hey, it will have done something, and something is the general result you’re looking for, right? So go nuts - install it. See what happens. You might even beat the odds and get away with it.”

All I have to add is that I think it is very irresponsible to even think that a master cyl. designed for a Silverado be expected to effectively stop a heavier vehicle especially since that heavier vehicle has a higher payload capacity too! please let us know when you intend to test your theory so I and the ones I care about have time to clear the road for you.

I’d be reluctant to try this type of brake modification. The braking system’s fuction is a balance between the size of the master cylinder and the size(s) of the slave cylinders (the brake cylinders), combined with the rotor and pad sizes, the booster, the proportioning valve, and other factors. Changing just the master cylinder to soemthing different could have an unpredicatable affect on the function of the brakes. Assuming a larger MC allowed you to push greater hydraulic pressure to the wheel cylinders, how will they then react? The pressure in the wheel cylinders is held by a “square cut O-ring” around the pistons…will that reliably withstand extra pressure?

Dual boosters are another issue of concern that others have adequately addressed. If connected in series they should yield something greater than one alone, but if connected in parallel they may not. Either way having them both there doubles the chances of a failure due to leakage.