Davidmij - Disc Brake Conversion & Rim Problems

[YellowRose]

davidmij - Disc Brake Conversion & Rim Problems

David, according to the 1959 Shop Manual (which you should have, if you do not have one), Table 3 - Manifold Vacuum Gauge Readings, on Page 1-11, shows that the gauge reading on a 352 engine with an engine in normal condition, should be 19-20 inches.

[jopizz]

The best way to test your booster is with a vacuum pump. That way you can see if it holds vacuum and releases correctly when you depress the pedal.

[davidmij]

Thx guys, I don't really know how to use this vacuum pump, but here's what I did. I hooked up the rubber hose to my manifold and it read 13 to 14. An article on SecondHandGarage.com said "the readings following will be typical at sea level. In general, subtract one inch for each 1000 feet above sea level." I'm at 6500 feet so that would mean the reading is 19 to 20. That's what it's supposed to be. Do you think the 1000 feet rule is accurate?
I also hooked the line to the booster and pumped the trigger - it went up to 15 and then no more. When I pressed the brake it released the pressure just like it should. However, each time after that it would only go to about 5 - weird.
I checked my 1992 F-250 460 at the booster and it read 16 to 17, but I don't know what it's supposed to be. I don't have a manual for the truck and I couldn't find it on the web.
Any help/siggestions would be appreciated.
regards, Dave J

[jopizz]

When you are hooked up to the booster and are pumping it you are generating vacuum; the same as your engine would. When you say that you are getting only 5 after pushing the brake does it stay at 5 or does it go down to zero which would indicate a leak. After you push the brake and release the vacuum it should go back up to 15-20 again when you pump it. That's probably as much vacuum as you are going to generate. Is your brake pedal returning to the full up position. It sounds like the booster is not closing all the way; either because it's defective or your pedal linkage is hanging up. As far as the readings go I'm not that familiar with the whole sea level thing but 18-22 is normal for most engines.

[davidmij]

What I meant was that the first time it pumped up to 15 - then I pressed the pedal and it released the air. Then I tried the same exact thing again, but it would only pump up to 5 - then I pressed the pedal and it dropped again. Of course it made less of a woosh sound when it was at 5.
My engine smokes, and I'm pretty sure I have blow by - that would lower my vacuum pressure but I don't know how much - but have any of you ever heard of blow by causing the power brakes to not work at all?
I guess I should call them back tomorrow and ask just how much pressure it takes to work the booster.

[Astrowing]

14.7 psi is nominal atmospheric pressure at sea level. 11.5 psi would be nominal atmospheric pressure at 6500 feet elevation, so there is about a 3 psi reduction. I would think an engine that generates 19 psig vacuum at sea level would be about 16 psig at your altitude since the reference pressure is 3 psi less. That is approximately what your 1992 Ford F250 is showing. If your are only getting 5 psi from your hand pump, it sounds like you have a leak. As far as booster vacuum, I think 16 psi is what they like to see. That's why they sell electric vacuum boosters for high performance engines that have low vacuum due to high overlap cams.

[davidmij]

Thx Jim. The bird was reading 13 to 14 so it would actually be about 16 to 17 by your calculation. I'll definitely call the brake guys tomorrow and ask if that is enough to work the brake booster. I'll also call the local Ford dealer to see what they know about my F-250.
- Dave J

[davidmij]

Hey everybody, just a little FYI. I talked to my local mechanic who's just around the corner (Herman) of "Herman's Garage". He said the most I'm going to see out of my car is 15 at this altitude. He said it drops about 1 for every 1100 feet of altitude. We are actually right at 6800 here, so that puts my vacuum right where it should be. With that I am shipping this unit back so you lucky people won't have to hear form me for a week or two!
thx for all your help.

regards, Dave

[simplyconnected]

I don't want to get off subject, but don't confuse 'inches of mercury' (vacuum) with 'pounds per square inch' (pressure). They are very different from each other.

In practical terms, if your air compressor puts out 100-psi of pressure, holding your thumb over 1/2" end of that hose would be really hard to stop flow. But holding your thumb over the INLET side of the pump would be very easy.

A 'perfect vacuum pump' (at sea level) cannot lift water 34 feet. In Denver, only 28 feet. I don't know of a well that's only 35' deep. That's why we drop the pump down the well, because the same pump can PUSH water under positive pressure hundreds of feet.

Most vacuum boosters are designed around 18-inHG (operating vacuum). Our vacuum boosters need a lot of space for that weak vacuum to exert hundreds of pounds on the M/C. That's right, hundreds of pounds of push, not psi. Depending on the diameter of the M/C piston, the output fluid transforms into psi.

The original boosters were sized to assist drum brakes. Here's a picture of an original 1960 booster:

I know it's a rough example but I needed this picture for the firewall bracket assembly portion.

This booster barely fit next to the A/C plenum and is much too small for disk brakes. Today, we use two-stage boosters. They are as large in diameter but longer. They also deliver 700 pounds of force.

[kuusamon]

The best way to test your booster is with a vacuum pump. That way you can see if it holds vacuum and releases correctly when you depress the pedal.

my suggestion is not meant as a joke, better say that first, but as a test vacuum pump, ever thought of trying a good vacuum cleaner for using of testing? I have not done it myself as of yet but I will run into the same problem soon when I start connecting and testing everything. A good vacuum cleaner will maybe come close to the vacuum and engine will pull ...maybe? its worth a try.

regards,
ron

[Astrowing]

Dave caught me on my units and he is correct. In English units: 1 inHg = 0.491098 psi, or 2.036254 inHg = 1 psi. Pressure is pushing, and vacuum is pulling.

At sealevel or 14.7 psi atmospheric pressure, there is 29.4 inHg in a perfect vacuum. At 6500 ft elevation, there is 11.5 psi pressure and a perfect vacuum would be about 23 inHg. The perfect vacuum numbers can not be obtained with an engine which is basically a vacuum pump and only gives a rough vacuum. It wouldn't be good enough to evacuate an AC system to dry it out for a recharge. This does tell you that there is a 6.4 inHg reduction in the available vacuum you can get at 6500 altitude, hence the 1 inHg / 1000 ft rough rule.

[simplyconnected]

Jim, you put things in a better perspective. So, 18-inHg is more like 8 or 9-psi of pull.

If our engines didn't already make vacuum, it kinda makes you wonder why Ford would use it. I mean, the oil pump puts out a whole lot more usable power and so does the power steering pump. Why mess with 9-psi?

The fact is, vacuum is very reliable. You can run out of oil or throw a belt, but the engine won't run without vacuum. Brakes command a margin of safety above everything else. Thanks.

[davidmij]

Wow, you guys are way too technical for me. I should have paid more attention in school. You should be working with the scientists here at Los Alamos National Lab! ;~)
So you agree that with the vacuum pump hooked to the manifold vacuum I should read about 1 less per 1000 feet of elevation? I really trust my local guy, he's old school and wouldn't BS me if he didn't know.
I know from experience that liquid under pressure is WAY more strong than air.
thx again, Dave J

[Astrowing]

Thanks, but I already have a job here in Houston at NASA. I've enjoyed working on mechanical stuff because it does give you practical experience which is extremely valuable. Theory only takes you so far.

Yes, I concur with the mechanic. If you were to put your gauge on your engine in New Mexico and it shows 13 inHg and drove it to Houston which is basically sealevel, I would expect to see 19 inHg on the gauge.

[davidmij]

Well, good news and bad news. I sent back the M/C setup to "abs power brakes" and they found one of the diapragm's to be bad. They shipped me another (it's on the way). However they refused (so far) to reimburse my $22 shipping costs to sent back a product that they sent out without even testing. Pretty lame if you ask me. I have an e-mail in to the owner of the company and I'll call to talk to him later today. I believe this is the same company that OID sells through. More to come.