vacuum brakes not holding any boost

[GNVYUS 1]

Read the OPs question, where does he ask about stopping at 140mph or vac brakes stopping his GN at all?

Jack uses a transbrake on his TH400, so ya, Jack isn't holding boost with the Vac brakes either.

 

[TurboDave]

Hurry someone tell Jack Cotton that the VAC brakes on his car are crap and he has to take them off and go with Hydroboost otherwise he cant lauch and stop properly.

No way VAC brakes hooked to whatever size rotors, calipers, disk/drum/whatever could stop a 140 plus MPH car long less a stock GN....

Nobody'd disputing their stopping power. They have great stopping power, it's the lack of clamping force to hold against boost on the line where they lack.
It's like putting disk brakes on the back axle and expecting them to hold good boost also. Aint going to happen. Ya, they can stop on a dime, but no holding force.

 

["Turbo-T"]

I think what everyone has summed it up to is a hydroboost is not only used to stop the car but also used as a poor man's trans brake.

 

[vicious6]

I have to agree. My GN has vac conversion on it (was that way when I got the car). Will not hold boost for crap! There is a reason every turbo car has a alternate form of braking. It is called pressure rather than vac. The accumulator does just what its name implies, it accumulates pressure to hold the brakes. I have even thought of line locking front and rear brakes and wiring to one switch so when you are ready, just flip the switch! Just an idea.

 

[grandnat86]

I have vacuum brakes and I can easily hold 5-6 psi on the footbrake. I only have soft shoes too, stock wheel cylinders.

 

[boostingirl]

i hate my new vac brakes, i am also having the same problems,on top of that mine locked up both front wheels with new brake parts wtf.

 

[TurboDave]

I have vacuum brakes and I can easily hold 5-6 psi on the footbrake. I only have soft shoes too, stock wheel cylinders.

Not very much, but I suppose that's OK for street tires. Like I said they just don't have the clamping force, especially when you get into boost, and your vacuum reserve just isn't enough.

Plenty of braking power during normal deceleration, simply because you're creating max vacuum.

When you can easilly build and hold 15-18psi of boost on a foot brake launch (as I can) then we'll compare holding force.

 

[Red Regal T]

Obviously, choosing vacuum brakes is a trade-off. Instead of the expense of keeping a powermaster working, or investing in a hydro-boost system, the trade off is the relative ease of getting and replacing parts, and additionally their inexpensive cost. Holding 5 or 6 lbs of boost at the line, for those occasional trips to the dragstrip is more than enough for most people who post here. Now those who go to the track regularly and their prime interest is a low 60' time, those folks, by all means, should invest in the better braking system for that purpose.

 

[bison]

it's the lack of clamping force to hold against boost on the line where they lack.

What is needed is more rear pressure. The proportioning will be the same no matter what brakes are on the car if the valve isnt changed. Therefore an adjustable proportioning valve will help a lot. Larger wheel cylinders will help too but make wet pavement stopping a problem. Ive seen rear shoes that were a problem as well as glazed drums. These things happen from burnouts with use of a front line lock.

 

[bison]

i hate my new vac brakes, i am also having the same problems,on top of that mine locked up both front wheels with new brake parts wtf.

If the brakes locked either you forgot to take your foot off the brake pedal or there is/was debris in the hydraulic lines or you have failing brake hoses that act as a check valve. The correction is to replace the lines/hoses/calipers and bleed all the old fluid out and replace it with clean dry fluid. You replace from the master cylinder to the wheels. Not from the calipers to the master cylinder or you may have continuous problems with dragging brakes as debris is dislodged.

 

[bison]

Hurry someone tell Jack Cotton that the VAC brakes on his car are crap and he has to take them off and go with Hydroboost otherwise he cant lauch and stop properly.

No way VAC brakes hooked to whatever size rotors, calipers, disk/drum/whatever could stop a 140 plus MPH car long less a stock GN....

To be honest stopping from 140+mph without a chute is a big chore at most of the tracks local to me. I know im on the brakes really hard at the end of a pass.

 

["Turbo-T"]

Now those who go to the track regularly and their prime interest is a low 60' time, those folks, by all means, should invest in the better braking system for that purpose.

Why not just install a trans brake??????

 

[turbo nasty]

I concur Bison.The Pumpkin does have a larger Baer kit on the front and yes hes uses the TB for launches.
The pic was just for illustration purposes to show that vac is out there in faster cars.
I get phone calls and Pm's of folks wanting to jump om the HB wagon when most simply dont need it. Most do the swap and say wow the pedal is higher and it stops just as good and now I have money to put toward an alky kit, exh, etc. Or wow the vac conversion was less the the flipping Accum or sw and i have a lifetime warranty to boot.

HB....
I have driven alot of cars with them...A few issues that arise when they start going with the HB IME is pulsing steering as you apply the brakes while turning the wheel like when pull in a parking spot or low speed manuvering....you can feel the steering slow or jerk a little as you press the brake, leaks, front brakes dragging on S10 front swaps, etc

Alot of folks breeze through these threads and get VAC sux peroid. Ive said it before and ill say it again if you are launch off the footbrake/e brake with 10 plus psi IME the backing plates get bent eventually...been there done that as well as sliding sideways up to a stoplight due to the S10 WC rear upgrade.

More that one way to skin a cat...... Put these Vac/HB/PM threads up there with what type of oil do I need to run in my GN. :biggrin:

 

[Red Regal T]

Why not just install a trans brake??????

Exactly! But don't ask me. I'm happy with my inexpensive vacuum brakes. I didn't install a hydroboost or insist on repairing PMs. :biggrin:

 

[Nick Micale]

These "my vacuum brakes won't hold boost" threads always amaze me?

The stock brakes on a Regal are all the same no matter if they are vacuum, PM, Hydroboost or manual, difference is the manner in which they assist the leg muscles. The calipers, shoes, pads and cylinders are all the same, so they should ALL have the same holding power.

When brake stall boosting, the rear brakes MUST keep the rear wheels from turning or else the car will move forward even if the fronts are locked. Since the fronts do 75% of the braking, the rears are usually neglected and not even up to their 25%. :frown:

Since the vac brakes are applied BEFORE boost is built, and there is reserve vac as well, the vac source is no longer a factor in braking effectiveness, it is mechanical effort from the time they are applied.

In the early years before most of our race cars went to trans brakes, we used vac brakes and were able to hold up to 8-10 PSI boost when the rear brakes were properly fitted to the drums, and bled/adjusted properly.

Biggest problem in most vac conversions I have seen is air in the lines.

Of course tires, street or track surfaces, converter stall, turbo and other factors determine how well an individual car will hold boost.

 

[Timbuick]

I'm sure you are right Nick, but i went to the drags with my GN for the first time today. i have vac brakes and recently replaced rear brakes and drums and adjusted them for good contact.
At the tree today I could hold no boost and if i did I started to push through the lights long before boost started to build.
Net result? 14 second et and pathetic 60'.
Would a line lock allow boost to build?

 

[86brick]

Before I did the Vacuum swap on mine with the stock Powercrapster I could hold 9-10lbs of boost and after the swap I had the same results, 9-10lbs of boost on the street with plain old radials. Just as Nick stated, if they are bled/adjusted properly you should have no problems or you might even be able to hold a little more. The Vacuum conversion was easily one of the best things I did to my car and made me feel much safer after having a close call.

 

[Timbuick]

That's interesting. My brakes are excellent for daily driving, but will not hold boost. I wish

 

[Nick Micale]

drum brakes are a problem..

The issue with real drum brakes is that it is difficult to get full contact between the shoe and drum?

Replacement shoes are usually a little oversize to compensate for drum wear and clean up on a lathe. But the "circles", or diameters of the drum and that of the installed shoes are most likely NOT the same when contact is made.

In the "good old days" before OSHA and others :frown:, we had a machine that would "true" or arc the shoes for an exact fit. In our city I only know of only one brake supplier that still has a working machine.

The alternate method by hand takes a little work, but is worth it. Remove the drums and see what the contact pattern looks like. Have some sturdy, rather corse sandpaper available, and work the shoes to fit the drum.

May take a couple tries with the drum on and off again to get a good close fit.

While the wheel is off, good idea to bleed the cylinders as it will only take a minute.

Like I said, if the rear wheels turn under boost, car will move. :biggrin:

 

[Nick Micale]

..........At the tree today I could hold no boost and if i did I started to push through the lights long before boost started to build.
Net result? 14 second et and pathetic 60'.
Would a line lock allow boost to build?

Tim, I just noticed you have a stock turbo with modified heads and a different cam.

My guess on your problem, if you have a stock torque converter it is WAY too tight for that combo.

At the stock stall it would be coupling too low, and does not have enough slip to develop need boost which is now at a higher RPM as you have dramatically increased the air flow into the engine.

What was your MPH in the quarter mile today?