How Much Vacuum Should a Good Brake Booster Hold?

[Ford F834]

A larger diameter bore provides more volume for larger slave cylinders and/or dual piston slave cylinders… BUT it will require more pedal effort than a smaller diameter master. Up sizing to a larger bore master will make for a firmer pedal and less travel. This is why the heavier F250 and 350’s had dual diaphragm boosters and the brake pedal pin was closer to the pivot to increase leverage. Hydroboost systems have the pedal pin even closer to the pivot than either of the vacuum brake systems and have the largest master cylinder bore. Also note that in addition to pedal pin locations, the different bore master cylinders do not all share the same mounting bolt span. They don’t directly swap. I personally prefer to keep the pedal, booster and MC matched per factory application.

I put a F-Superduty hydroboost in my F150. The pedal has very little travel and extremely effective stopping power. This is a bolt on upgrade, but the parts are not easy to come by.

[viven44]

I will make a video as this is a good one... but I was able to use my HVAC vacuum pump to pump down the good brake booster to 25" and even lower and it held that vacuum...

My bronco's brake booster on the other hand, even the HVAC pump couldn't go below 5" and it didn't hold that 5" so no surprise there is a big leak and why my brakes are so horrible. On my Bronco only 5" real-time engine vacuum is available for braking use. No hissing noises or anything like that or loss of idle characteristics while braking. Very strange.

[grumpin]

My 86 F250 had stiff brakes and my front calipers were rusted up and barely moving.

Might check the calipers and wheel cylinders too.

[viven44]

Here is a video of me testing it with a larger vacuum pump

https://youtu.be/07t-_R965Vc?si=xj1otOndKB2Ki1Ob

[mat in tn]

well done on the video and showing the fail vs sound.

[Gary Lewis]

I like it.  But how are the brakes on the Bronco now with the new booster?

[viven44]

Thanks Matt!!

Gary, TBD Saturday. I am hoping to get that done before it gets toasty outside... but when its cool enough for the mosquitoes to be out and have a meal

Also hoping to get the master cylinder out and set it out of the way.... that would be fantastic if I don't have to open up the brake lines.. All those coils of brake lines I assume are handy for this purpose to move around the master cylinder just a bit.

[viven44]

New booster did not make a world of difference.. if I thought it did it is probably placebo effect

Quick in and out swap though.... had cylinder out of the way…. worked early morning when it was still feeling good.

If any positives, the engine vacuum is improved slightly. Over 20" while idling on ported vacuum and solid 22" in manifold. I may also have more braking power in a stalling situation now than I did so I'm happy I replaced the booster.

[Gary Lewis]

Well, at least you know you don't have a problem and you aren't sucking dirty air into your intake.

[viven44]

Yes very true on both!

[viven44]

Well the new booster did reduce the variables I was worried about. I always thought it was the booster but when I realized it wasn’t all of it I had the conviction to go after the next one which is the pushrod length. I have always errored on the side of small as I would rather have more pedal travel than dragging brakes but this morning I went ahead and 3D printed the jig to make a precise adjustment.

My brakes are still 'soft' but thats probably what I get for using $15 Raybestos shoes and pads.

It really firmed up the pedal though. I was at 0.95” and I changed it to 0.98”.

From 78 Truck Shop Manual

[Gary Lewis]

That's cool!  If we had a page to share 3D plans would you share that?

[viven44]

Absolutely, in fact you can download from here
https://www.tinkercad.com/things/d9m2WgRlwyW-brake-booster-adjustment-jig?sharecode=e7ODS2c4UAyOEiNQSTgsCIijfWua1H6tCsFSNFyDPh8

[Gary Lewis]

Vivek - If it is ok I'm going to download that file and make it available directly on here instead of through TinkerCAD.  The reason being that people have to sign up to TinkerCAD to download it and this will make it easier.  And it also ensures that if something happens on TinkerCAD we have it.  Is that ok?

And I'm going to use the diagram out of a Bullnose FSM just to assure people that those are the right dimensions.  But, they are.

[viven44]

Gary, of course. It is open source!

I did observe that on my 78 Brake booster the 2 15/16 dimension was a little too big to fit in between the studs. I had to rotate the jig to use it. If I had to redo it I would shorten that side just a bit.

The 2 15/16 was right on the money as-printed

Appears to have the same problem on the bullnose brake booster based on a quick eyeball

This is old cylinder from the 78. Clearly too big on it

[Gary Lewis]

Do you want to revise the drawing with the smaller width and then send me the file?

[viven44]

Yes let me revise and print and test it …

[Gary Lewis]

I have the page up: Documentation/3D Printing/Brake Booster Pushrod Gauge.  And the illustration from the FSM is on there, as shown below.  But it shows that the surface of the booster isn't even between the studs.  So you'll want to ensure you catch the high spot.

[viven44]

Yes I saw that. On my reman 78 booster though that profile is slightly different than the core I turned in yesterday. There is a “bump” closer to the center and in ford style I’ll guess there were small variations between applications. The incoming bore on the cylinder will also need to be measured vs the old one. That’s why I printed close to 0.98” so it will error on the small side.

I’ll measure between the holes on the cylinder and get it as close as possible so it will fit but not be too small.

[viven44]

Well I have good news. Instead of assuming the bullnose and dentside are the same I decided to check the gauge on the 86 F250 parts truck which is a highly original truck and assumed all original parts

The gauge works as-is on a bullnose 3/4 ton. I can’t say if it’s a 1/2 vs 3/4 ton difference or if it’s dentside 1/2 ton vs bullnose 3/4 ton but my 78 bronco needs 2 9/16 instead of 2 15/16

Pics below from the 86 F250 3/4 ton parts truck