Adding/Running a Power Brake Booster with Low Engine Vacuum

[Original77]

I have a 1977 Bronco with Disks up front and Drums in Back. I do not have a Power Brake Booster, I'm not sure if one was ever installed on the vehicle. I have about 11 PSI on the vacuum meter when testing at idle, or when the Bronco is in Drive Gear with the brakes applied. I live at 5800 feet altitude. From what I've read, my engine doesn't have enough vacuum to run a Power Brake booster successfully, some even recommend running a vacuum canister or to even run a separate vacuum pump to run the power brake booster. Has anyone been successful running a Power Brake booster with such low vacuum and if so, how does the peddle feel ?

Thanks, O'77

 

[DirtDonk]

Hey there. Welcome to classicbroncos!
And wow, 11 inches, eh? That's really low. Is it stock, or is there an aftermarket cam at fault here?
If stock, even at that altitude I would think (hope) that you could raise that number substantially by different tuning. I could be wrong, but I think it's doable.

I'm interested too, to hear what others experiences have been (just in case I haven't heard a good one) but I'm going to say right now I doubt you'll hear of anyone having good luck. Even the car guys have been complaining about that kind of thing for 50 years or so. And they have much bigger booster-to-brake ratios than we're ever likely to see on an EB.

As for original or not, while most EB's by '77 had the booster, it was still an option. So there are some running around just like yours.
Is there a factory looking spacer thingy sandwiched between the master cylinder and firewall? There would have been one present from the factory with manual brakes from all I've seen. Maybe about 3/4 to 1 inch thick or so, if I remember.

Good luck. Hope the answers are helpful when they come. A lot of members here are likely to recommend the more expensive hydro-boost option in your case.
But let's see if we can't get that vacuum up a bit anyway.

Paul

 

[Digger556]

Low engine vacuum can be helped with a vac pump. They are fairly common on newer cars. I know for sure Jeep JKs use them. An OE quality pump runs about $100 plus some plumbing. I would start there, as it's cheaper than hydroboost.

That being said, I love my hydroboost and would consider it a must if you have larger tires.

 

[pcf_mark]

I would use hydroboost because it is just so much better than vacuum brakes.

That being said I have experience with this pump on a car that had 6" of idle vacuum (it was ridiculous) it was pretty quiet (NOT silent) but worked very well giving good feel and performance.

https://www.summitracing.com/parts/cvs-vp555

 

[Original77]

Its a stock Bronco, I recently swapped out my power valve on the carb to a 5.5 and it runs so much better (The PV should also be picked according to engine vacuum). No gasket/spacer on the manual master cylinder, I don't think the Booster option was ordered when the Bronco was new. The manual brakes do work OK, you just need a little more leg than normal to stop. Thanks for the replies.

 

[Original77]

I believe the altitude is the biggest killer of my vacuum, sources on the internet say you can lose up to 1 inch of mercury for each 1000 ft above sea level, roughly. So at sea level, I'd bet I'd close to 16-17, which would be normal range for power brakes.

 

[Starman]

The PO of my 67 put in a separate vacuum pump. I'm sure he did this because of the cam. I can get pics and model number of the pump if you would like. The pump is run off a relay. If the key is turned the pump is running even if the engine is not running. I must have leaks in the booster or hoses because the vacuum pump cycles on and off about every 15 seconds. The pump kicks in when I step on the brakes. The pump is also wired into a pressure switch to make it cycle when needed.

 

[TOOLMAN]

I'm at 5280' and my power brakes work great. I've even had it up to 10,000' with no ill effect. I only put on power brakes a few years ago having run no power for the first 20 or so years (I had to to clear the EFI plenum). I love it! I'm not a huge believer in hydroboost...seems like a little bit of overkill. I might feel different if I were running a slush box or 36's!

 

[hyghlndr]

Hydroboost would be the best option.

 

[Rustytruck]

I would check the slack in the timing chain and check where your initial timing is. Make sure to get as much vacuum you can get.

 

[Original77]

Hey TOOLMAN, do you know how much vacuum your engine is creating? Living in Denver, I would think you are lower than average too.. I'm just wondering how low on the vacuum I can be at altitude and get by using a power brake booster.

 

[gr8scott]

My Edelbrock Performer-Plus cam is rated for 16" @ 1000 RPM. I get about 12" at idle. It ran a vacuum booster just fine.
I live at 3000' and have also run to 10,000' several times without any ill effects.