Technical big cam no power brakes

specs.ot truck at work .67 chevy with a built bbc auto trans and factory power disc brakes.have no specs on the cam but it only has 5" of vacuum at idle.obviously too little for the power brakes so i got a vacuum canister to help but no difference. the booster is working (holding vacuum).is the 5" too little to work even with a canister?.should i get a vacuum pump?ideas? thanks

 

Ditch the power brakes. Drop to the next smaller master cylinder bore,and drill a new hole in the pedal, to get to 6:1, or higher, if there is not already another hole there.

 

Sounds like it my need a tune up if only 5 inches.
The canister will only help if you can get the vacuum higher.

 

You can also convert to a hydro-boost setup.

 

+1
we did this on an ot 67 chevelle with a small block with a pretty healthy cam, i think it pulled about 11" at idle. now granted we put 13" rotors on it, but if you weren't careful it would toss you through the windshield.

 

Maybe try more initial timing to see if the vacuum at idle will increase. 5" seems awful low unless its really a wild cam.

 

thanks guys gonna try some things today at work to see if ican get the vacuum up but any more suggestions are appreciated

 

If the cam is too big, no amount of tuning will get the vacuum up to power brake levels. I'd guess if it's at 5" now, it sure wont triple, and something much closer to 15" will be needed for power brakes. A booster can wont raise vacuum, it merely gives more volume, so they're a waste of time for low vacuum situation.
Go manual or hydro boost and you'll have plenty of braking.

 

Hi. Definately no expert on this but many mechanical fuel pumps can be purchased with vaccum ports that are not dependent on manifold vaccume. The fuel pump action creates the vaccum.
If you are running mechanical pump see if there is one avalible with vaccum ports.
If you are running electric pump could you retain the electric and just add a mechanical pump with vaccum ports & use it for vaccum only? Or there are electric vaccum pumps avalible also.
I know the mechanical fuel pumps with vaccum ports were great for the older vechiles with vaccum wipers as the wipers vaccum was not then dependant on the constant varrying manifold vaccume.

Just might be an easy solution. Then it might not. Hopefully others will no comment on this.

Jimmie

 

You could run a electric vacuum pump. I had the same issue. Mine would work much better with the vacuum line hooked up to the booster rather than disconnected and plugged (psuedo manual) but I found a clutch return spring helped bring the pedal back and even with only 6" my brakes are fine.

 

How much might things improve with either a larger diameter booster or a dual-diaphragm booster?

 

never had a great pedal (4 wheel disc) in my 50 international p/u with a sbc. tried everything. on the recommendation of a friend, I swiped a hydro assist set up out of a junk late 90's astro van, 2 1/2 hrs. later, I had the best pedal ever. it stops on a dime, give's ya a nickel change!

 

I've just completed a Hydroboost install recently on my El Camino...I highly recommend this swap!

 

Your Engine may only have 5 inches at idle but under deceleration it is way higher do you have a one way check valve in the system? It needs to be between the engine and the booster and canister. Unless it is a very large canister is only good for one stop.

 

Interesting convo here. My 70 torino 351c with a big cam runs 7 in and barely enough for brakes. I remember having cars back in the 70's with hair triggered boosters and was told it was touchy because of a pin hole in the booster diafram. Never made sense to me but i'd like to find one of those malfunctioning boosters now. Put ya through the windshield.

 

I used a electric vacuum pump on my bus conversion. I just wanted a back up incase the engine died going down a hill. Anyway what im getting at is I used nice check valves from a big rig. There brass and threaded on each end, 3/8 pipe i think. Made for preasure but they work just fine turned around for vacuum.

 

my 56 Merc Amblewagon [ ambulance ] has vacuum wipers that would *stall* on acceleration -- cured that w a late model ambulance 12v electric vacuum *suction* pump -- works GREAT!

hasta be switched on & off tho -- gotta remember

Back in the day, 50s - 60s, Trico made the *Electro-vac* which kicked in when engin volume dropped on accell to keep the wipers movin - - *it* was adjustable too Beulah had onea thos -- got anuther somewheres out in the barn --