Vacuum Brakes...Myths, Legends, and Wives Tales

[John Larkin]

Ok, I can only hold 2psi. That's it. From what I'm told, the PM has like 1800psi braking pressure while vacuum has 900-1000psi. I'm also told that if you increase the piston size on a vacuum master cylinder, you lose braking pressure as well. So there are trade-offs. PM = unreliable due to parts that go south unexpectedly. Vacuum = less holding power usually.

Maximizing a system sounds great. Who radiuses shoes these days? I thought that went out with the asbestos scares. I've bled mine twice and still have a great pedal but low torque holding power. I have no idea how to make it better other than modifying the system to use a jumper valve to double the pressure to the rears. My converter stalls at 3200. Woody mentioned it could be a factor.

Hey, say it, spray it, whatever. Get your thoughts out here. This is good stuff.

 

[BoostKillsStres]

John,
I've got the factory PM that just died 2 days ago w/ no warning, I went to jump in the car and the brake light stayed on and the pump won't stop running and isn't pumping any fluid. I ran through the daig. chart on gnttype.org and am replacing the enitre unit from autozone for 143.99 w/ exchange 61.00 core which I'll probably keep for the good accum. and pressure switch. I feel like almost 15 yrs of service w/ only one new accum. isn't bad.

I can hold almost 7psi and thats not pressing real hard on the brakes. I think Scott says to stand on the brakes until your face looks like then see how much you can hold. On the rear shoes since its hard to feel the correct drag on a posi when adjusting here's what I do: Pull the drums and lightly sand the shoes to remove the glaze then set the drag to what you think feels good then drive the car for a few days then pull the drums and check the wear pattern and adjust more if needed to get good contact and holding power. A good adjustment here also made a huge difference if pedal feel and height.

HTH

 

[rastaz]

You're lucky you can hold 2psi... I converted to vacuum once... After an embarrassing trip to the track I fixed my PM and now I'm happy again launching at any psi I desire and not looking like a constipated clown trying to do it .

Here's another myth "TTAs can hold boost on their vacuum brakes" WRONG! I have to reef on the e-brake which lets me hold about about 5psi. Maybe I can convert it to a PM...

 

[FlyinGN]

I can hold at least 7 psi with my vacuum brakes.

 

[TR Custom Parts]

Transbrake!

 

[87LtdT]

Don't know anything about vacuum brakes but I have had persistent questions about my PM system...which I thought was the reason I could hold no more than 2 psi until rear tires broke loose...got a bit of data that may be pertinent

The setup...new PM assembly, brakes bled and bled and bled...soft shoes, 7/8" dia brake cylinder, shoes adjusted, faily newly turned AL drums, and a line lock that allows the brakes to stay cool during burnout...MT Streets.

With stock D5 converter...1-2 psi and rear tires broke loose

With 3000 stall ATI converter...5+/- psi and rear tires broke loose

With 3000 stall Yank converter...and street tires...7-8 psi and probably more and tires have not broken loose.

No mods to the engine during these comparisons...

I asked many times on this forum what was wrong with my brakes...got lots of responses with a few advising the converter may be too tight...I think this few were correct.

HTH, Bob

 

[Nick Micale]

Originally posted by John Larkin
Ok, I can only hold 2psi. That's it. From what I'm told, the PM has like 1800psi braking pressure while vacuum has 900-1000psi. I'm also told that if you increase the piston size on a vacuum master cylinder, you lose braking pressure as well...........Get your thoughts out here.

The information that the vacuum master cylinder can only hold 1000 psi is wrong! I personally have seen 1800+ psi to the rear brakes on 2 different cars with a gauge installed when I was on the pedal. Neither had a Powermaster.

Without going into lots of details that was just in another posted subject, the blame, or credit, for holding boost is not in the mode of power assist. It is in the braking system itself and the build up on the vehicle. If you can only hold 2 psi, start checking all related items.

You also say nothing about your front brakes, that do 75% of the braking, having been recently serviced. Glazed rotors do NOT hold well at all, seem to stop OK, but will not hold under pressure. Seen them get glazed in one or 2 trips to the track, or in a few hundred miles of driving. This was the problem in one race car I recently fixed. [Stock class car with no transbrake allowed.]

My approach, forget about the power assist, and look elsewhere as mentioned to solve your problem.

 

[Nick Micale]

Originally posted by rastaz
Here's another myth "TTAs can hold boost on their vacuum brakes" WRONG! I have to reef on the e-brake which lets me hold about about 5psi. Maybe I can convert it to a PM...

Let's not put out mis-information here Rob. The reason the TTA cannot build lots of boost is the result of the rear disk brakes, NOT the vacuum booster. Put in a drum brake rear and it WILL work.

This is not an opinion, but based upon what was actually done with a TTA.

 

[JASON]

Originally posted by BuickGn Boosted
Transbrake!

GO WITH A BREAK AND LEAVE ABOUT 15PSI

 

[87LtdT]

Nick, your post is not applicable to mine...I never pushed thru the lights only rear tires began spinning. My point was that the converter has an affect on brake holding ability...I was trying to get the brakes to hold the same 5-10 psi I saw posted everywhere and had done all of the hardware things short of adjustable proportioning valve..with no affect.

A change of converters did the trick....which to means my brakes are about as good as they are gonna get. Guess I could install a second long shoe and add the prop valve...but I think they are good enough...where before I was always fighting them not realizing it was probably my converter stall.

Bob

 

[rastaz]

Originally posted by Nick Micale


Let's not put out mis-information here Rob. The reason the TTA cannot build lots of boost is the result of the rear disk brakes, NOT the vacuum booster. Put in a drum brake rear and it WILL work.

No mis-information here Nick. People seem to think that because TTA's came with vacuum brakes that they can hold tons of boost. Yes a drum brake rear would help but my point is once you run out of vacuum, you're done and that's where the e-brake comes in handy.

Obviously there are a few out there that have had great success with vacuum brakes but I see many more that are dissapointed with the performance of this system at the track. IMO the mis-information out there really lies how to set up vacuum brakes for racing rather than the system itself.

 

[jsta6]

I love my vacuum brakes. I can easily hold 5-7 pounds. Anyone who is unhappy may want to check their check valves (with a REAL tester, not by blowing and sucking) and you may also want to look at the summit racing.com vacuum reservoir. I personally know 6 people in south florida that have converted and they like it much better then the PM units.

 

[Nick Micale]

Originally posted by rastaz

Obviously there are a few out there that have had great success with vacuum brakes but I see many more that are dissapointed with the performance of this system at the track. IMO the mis-information out there really lies how to set up vacuum brakes for racing rather than the system itself.

Just trying to keep the discussion to what I personally know and have experienced and that includes setting up LOTS of cars for the strip from the 13's and faster.

When sitting on a starting line and building boost, the brake system does not know if it it is powered by vacuum, electric motor, hydraulic or a leg muscle. The brake holding capacity is a function of the hydraulic parts doing their job and how well the friction surfaces will hold.

So this now leaves the rest of the combination, turbo, converter, tires, track surface and so on. There are as many owners saying they cannot build boost with all the various power brake assist types. It is not the type they use, it is the car is not prepared correctly, or the build combination is not right.

Before this board was in exsistance, lots of guys were complaining the Powermaster would not hold boost and went to vacuum. This worked for them. The rest of that story is they also were doing a brake job along with the conversion and it showed up with better holding power. Could have done the same with a PM by doing a brake service.

The guys going into the 11's find it is more consistant and actually easier on the car to go with a trans brake when the desired launch boost is over 8-10 psi. But any good prepared brake system on a T-R should have no trouble holding 10 psi.

 

[1of1547]

Nick, what was the other subject post you were talking about? Was it on vacuum brake conversions or something else? I am getting ready to do this conversion.

Thanks

 

[Nick Micale]

Originally posted by 1of1547
Nick, what was the other subject post you were talking about? Was it on vacuum brake conversions or something else? I am getting ready to do this conversion. Thanks

It was actually on holding boost.

The GN/TTYPE web site has a detailed procedure for the vacuum conversion in their tech resources. The hardest part is changing the pedal. Fortunately, I have younger, thinner guys around to do that!

 

[TurboJim]

Originally posted by Nick Micale


Let's not put out mis-information here Rob. The reason the TTA cannot build lots of boost is the result of the rear disk brakes, NOT the vacuum booster. Put in a drum brake rear and it WILL work.

This is not an opinion, but based upon what was actually done with a TTA.

Yep, Nick is 100% right. Drum brakes stop better, they just dont dissipate heat and will fade unlike a disc brake. Read again, drum brakes hold better by design. They jam the primary shoe into the drum which causes the whole assy to "servo" and bind up the drum. Its like putting a chock in the door, it wont move.

I dont know how much boost I can hold, I do know I can hold the car without creeping until the rear tires break loose. Thats somewhere around 10psi and I have vacuum brakes as well.

 

[TurboJim]

Originally posted by rastaz


No mis-information here Nick. People seem to think that because TTA's came with vacuum brakes that they can hold tons of boost. Yes a drum brake rear would help but my point is once you run out of vacuum, you're done and that's where the e-brake comes in handy.

Well thatrs the thing. People dont think about what theyre doing and make a judgement based on what they did ewrong.

On a vacuum brake car without a vacuum reserve, it may be necessary to build some vacuum after your burnout. This is done by simply putting the car in N (should do that to clear the motor out anyway) and blipping the throttle a couple times. On the engines decel, vacuum is its hjighest. You may idle at 12" hg, but after you blip the throttle, a few bursts of 20" hg will enter the brake booster, if the check valve is working right, this 20"hg should remain in the booster until the next application (in the beam). Now if you're in the habit of pre staging, then bringing boost up and bumping in, yes, you WILL lose vacuum and lose assist, and wont be able to hold any boost without REALLY mashing the brakes down.

What I recommend is bumping in at idle, put it in N real quick and blip the throttle, let it return to idle, put back in D and roll the rest of the way in, as the staged light comes on plant the brake and bring boost up. While I havent been on the track in years, thats what worked (and still works on the street) for me

 

[FlyinGN]

yup Jims right on how to really hold boost. I learned that trick from a Syclone driver about 10 years ago

 

[Phatman]

I converted and have been happy with the whole set up. It was a pain in the a$$ job but no more problems with my brakes and holds boost to around 7 or 8 lbs depending on lockup.

The only draw back I have found is with street racing. It can be a real eye opener to have to brake from boost suddenly and not have brakes for a split second...seems like a life time.

 

[azgn]

I'll say it again...if your vacuum brake system is working properly and your converter and turbo are properly matched, you "should" be able to hold at least 10 lbs....if I can do it (and I can), believe me, anyone can do it! (I am a klutz launching off the foot brake.....the trans brake is the method of choice for me!)

what Jim says about building vacuum after the burnout is very important.......and if the turbo and converter are matched, you should be able to go into the staging lites at just above idle and get your 10 lbs before the last yellow (after the other guy stages)

if not, something is amock with the combo...plain & simple

turbo cars having to sit there and build boost(after the green) are sorta embarrassing at the drag strip, IMHO