Larger vacuum brake booster

[jauguston]

As long as I am being nit picky reference your last sentence you can pressurize a liquid but I don't believe you can compress it. All in good fun !!!

Jim - just returned from my grandsons 7th grade jazz band concert - grand kids are the greatest !!!

 

[yARIC008]

Pressure applied from the brake peddle will move the volume of a given diameter cylinder. Now if you have lets say a 1 1/8th inch dia cylinder it will move "x" volume. Now change the dia of that 1 1/8th inch cylinder to 1 4/16th and the same peddle movement moves a larger "volume" of fluid but at a lower pressure. In the end it just works better having a larger diameter master cylinder being pushed by the same foot the same distance. The increased need for peddle pressure is not noticed by those of us that have done this. Your information is taken in to concideration and I'm not offended because you are correct. I just know the larger master cylinder works because its pushing more fluid into the same space as the smaller mastercylinder can. Gas will compress but fluids wont. Its all just science. You can compress fluids but you cant make them occupy a smaller space. :Sly

Ya know... i did the bigger master cylinder mod, need to do the bigger vacuum booster mod probably now too... But anyways, at first i did have to really push into the pedal to get it stop normally and even then it wouldn't stop perfect... and now some years later i find that i have to keep quite a bit of pressure on the petal to keep the truck stopped. Like, when i'm at a red light and i'm just sitting there, sometimes if i don't pay attention my foot will let off just a little bit and i start rolling forward. Kinda worries me because you can't feel it it's so slow. Maybe my booster is going out, but it all started when i got the new bigger master cyilnder

 

[datkinsonsr]

icanfixall,
Interesting topic here. My $.02 worth, since the wheel cylinders will be the same, the distance their pistons will travel does not change. For the sake of argument, there is no fluid movement. It involves a change in pressure, force and a change in mechanical advantage. I ran some numbers using the diameters given for the master cylinders pistons and assumed a pedal force of 20# for this argument applied to both master cylinders. The first MC would produce a force of 1381#. The second MC would produce a force of 1483# Pushing the pedal essentially compresses a spring which is increasing force on the MC piston.(greater force output). These values just mentioned would multiply the force output at the wheel cylinders by a similar amount. My force input was very conservative.(Vacuum assist not considered). Under actual numbers, Will the seals at the wheel cylinders hold up to these increases?

 

[Agnem]

Jim, I'm glad your doing this experiment. All things being equal, and incremental improvement in braking, for an incremental cost is a worthwhile discovery. This way, folks who can't/don't hyrdroboost, have an alternative that is still worthwhile. I should have thought to mention that the 93 master cylinder would probably have to be used. It may also be partially responsible for the improvement in stopping power. I wonder if a 93' push rod could have been used, instead of having to fab up your own?

 

[jauguston]

Mel,

It would not have been necessary to update to the '93 MC. I could have slotted the stud holes in the MC and it would have fit just fine. I was having some unexplained fluid loss for some time that I could not locate so I decided to go ahead and upgrade to the '93 MC. The '86 and the '93 MC are both 1 1/8" bore so there is no difference in the amount of force applied with one over the other. In order to connect the lines to the '93 MC I had to get two adapters . One was a readily available Edelmann fitting and the other to go in the big threaded hole in the MC had to come from Ford. There book called it a spacer ($21.00). I now have it all together. Took it for a short ride this evening and the pedal pressure is definately less. I will not know how much better it really is until we go RV'ing again in a couple of weeks.

The issues with the push rod being too short and the stud spacing being too close together could possibly be addressed by more research in the parts application books at the parts house. There could be a 10 3/4" booster with the correct specs but I did not go back and look. That is for someone else to do. I did not figure out how to get the booster push rod that goes to the brake pedal out of the booster so I could not swap them. I have a TIG welder in my shop so rather than spend time trying to figure out how to get the pushrod out I just cut it off and extended it 3/4".

I am going to take a look at one other thing in the system. After a thorough bleeding there is still more pedal travel than I think is necessary, it feels a little spongy. I am suspicious that the '86 vintage brake line flex hoses are expanding under pressure. I had a similar issue on my Honda Goldwing motorcycle and I replaced the flex hoses with stainless braid teflon lined hoses and the brake became much more solid. Russell and Goodridge both make that type of hose. The harder you pushed on the brake the more brake force was applied without pedal movement. It was solid. I am now researching to see if I can find a set of three brake hoses made out of the stainless braid teflon lined hose. More later (-:

For those of you that replaced your master cylinder with one with a bigger bore -------------you went the wrong way if you wanted to create more braking force with the same amount of pedal pressure (-:

Jim

 

[TLBREWER]

For those of you that replaced your master cylinder with one with a bigger bore -------------you went the wrong way if you wanted to create more braking force with the same amount of pedal pressure (-:

I've been following this thread to see the outcome. From the very first time I read about someone putting a bigger MC in place, I thought the same thing. I'm no mathematician, but I worked on hydraulics some in my 20 years as an aircraft mechanic and now work with fluid dynamics in chemical distribution systems. Seems to me, all things being equal, more volume (bigger hole) equals less pressure. Less volume (smaller hole) equals less pressure. The part that I guess we can't measure at this point is the effect of the pressure on a given surface area (wheel cylinder).

Just thinking out loud.

Tom

 

[datkinsonsr]

Tom,
F=A x P, from this formula you can figure out anything in your hyd system regarding pressures, forces or the area of the pistons (booster diaphragm also).

V=D x A, this is good for finding volume displaced, area of pistons, and the distance the piston moves.

A=3.14 x Radius squared, will give you the areas of the pistons.

NOTE; volume has nothing to do with force output. Force output is a function of the force applied to the area of the piston, and the resulting pressure created acting on another piston. Said another way, the amount of pressure applied to a surface area of the piston determines the amount of force created.

 

[jauguston]

I was thinking about a way to explain the theory of how this all works last night and came up with a way to explain it that makes sense to me if not to anyone else (-:

We have three things that we are dealing with that can be consider as variables. For this discussion we will assume the pressure applied by your foot will be constant. Lets use a bottle hydralic jack as an example.

First we have the handle we pump the jack with - this would represent the booster. The longer the handle (bigger diameter booster) the more force we can apply with the same amount of effort.

Second is the piston of the jack (master cylinder)that is operated by the handle (booster) .

Third is the jack ram that does the lifting (the wheel cylinder).

The smaller the piston (master cylinder) operated by the handle (booster) the more pressure will be applied to the ram (wheel cylinder) and thus more push on the brake shoes or pads. The larger the ram (wheel cylinder) the more force applied to the brake shoes or pads.

I hope this makes sense to someone other than me!!

Jim - Retired with too much time on my hands to think about this stuff (-:

 

[icanfixall]

Well. This is sure a great topic. I'm sure glad we have so much imput with great backup statements. This is what makes this such a great site with all this information.

 

[jauguston]

One other point I forgot to mention in my last post. There was mention of volume of fluid flow by a poster. As mentioned by another poster flow or volume are not a issue that has a bearing on how things work. The front disc pads have .004 to .006 clearance from the rotors in the retracted position. The amount of fluid volume necessary to move the pistons and pads into contact with the rotors is very small even though the caliper pistons are quite large. Same in the rear if the brakes are properly adjusted it takes a small amount of fluid volume to move the pistons and shoes into contact with the drums. Once the disc pads and brake shoes are in contact with the rotors and drums there is virtually no further flow - just a increase in pressure as needed to stop the vehicle.


Things like air in the lines or soft rubber flex hoses will make the pedal soft but a properly operating brake system will have a pedal that is hard after the slack is taken out of things.

Jim

 

[icanfixall]

With the rear brakes out of adjustment MORE fluid at ANY pressure is needed to make them work. So, now we have a larger piston pushing more fluid volume to the out of adjustment rear brakes. This is why the peddle will slowly drop when we have stopped AND the rear brakes are out of Proper adjustment. The master cylinder is just doing its job by pushing fluid into any space trying to fill it. The fronts work first, then the backs. Now if the backs need adjusting it will take more fluid to push the wheel cylinders apart. The rear shoes need to be about .025 thousands from the drums to be in proper adjustment. How we can measure that is beyond me but thats the story. A larger mastercylinder will push more fluid per stroke but if you may at a lower perssure. We don't want to change the peddle fulcrum to make more peddle pressure on the larger cylinder but we could. So in closing "if" the rears are out of adjustment pushing more fluid to them will work. Ever pump the brakes and they worked better....

 

[jauguston]

Icanfixit,

I read what you just wrote three times and I will be darned if I can understand what you said. What is the english translation (-:

This whole discussion comes down to one thing, the more pressure you are able to generate at the brake pad and shoe with the same pedal effort the better you are going to feel about your brake effectiveness. More pressure = quicker stopping.

Jim

 

[datkinsonsr]

Well said Jim!

 

[jauguston]

Update - Long road trip - Happiness!!

I just got back from a 500 mile trip and what I had hoped for in braking improvement is there. To review - I replaced the stock '86 8 3/4" diameter brake booster with a 10 3/4" booster from a '93 E-350. I also installed a '93 master cylinder but the stock '86 MC would give the same result.

It is all subjective but I feel like the pedal pressure now is 50% of what it was for the same amount of brake force. The pedal has a nice "light" feel. Before the change when I made a aggressive near panic stop I was literally using all the strength in my leg. Now it is moderate pressure at most. I am grossing around 15,000 lbs. If your rig has a 8 3/4" booster this is well worth the work.

This is a simple swap. Using a stock MC the bolt holes in the MC need to be slotted about 1/8" on each side as the studs on the booster are 1/4" closer together than the '86 booster. A rat tail file will do it, it is cast iron and cuts easily. The pedal push rod needs to be extended, I extended mine 3/4" but it would have been better if I had extended it 5/8". The pedal is a little too high. Cut it off in the flat part right near the transition to round. I beveled the joints and TIG welded the spacer piece in.

I was able to talk the parts store in to taking the '86 booster as a core on the '93 booster so the total cost for the change would be less than $70.00 using the stock MC. Money and time well spent!

Jim

 

[krawlr]

Update - Long road trip - Happiness!!

[MrBurns] Eeeeexcelent! [/MrBurns]