Ford F834 Posted August 14 Share Posted August 14 I think this discussion on a Chevelle forum explains what's going on here. https://www.chevelles.com/threads/bench-testing-a-brake-booster.1097742/ So back to my problem, my brake suck. I need to stand on the peddle hard and it barely stops the truck. I've only driven this truck about 120 miles since buying it, and the brakes were always like this. I thought it was because the rear wheel cylinders were bad, but I've replaced those and the brakes still act the same. Front pads have lots of life left, and rear shoes are new and the adjustment seems to be good. I'm wondering if my master cylinder is undersized? A F250 should have a 1.0625" bore, and a F350 should have a 1.125" bore. My truck has been upgraded to a Dana 60, and F350 booster, but I don't think I can verify that the master was upgraded to the larger bore without pulling it off the truck to measure? Would the smaller master cylinder even bolt up? 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. Link to comment Share on other sites More sharing options...
viven44 Posted August 15 Share Posted August 15 That's interesting, but doesn't really explain why you can only get to 7" of vacuum. However, the internal valving they talked about may be the issue and since I don't understand it I don't understand why you can't pump up more vacuum. As for the master cylinder, my master says right on it that it has a 1 5/16" bore. It is from an F450 and works well on my truck, which also has a D60 up front. BUT, I'm using hydraulic pressure instead of vacuum for the booster as I have a hydroboost unit from that F450 as well. And you don't want to touch the brakes if you don't want to stop. They WORK! But a bigger bore size doesn't mean that it is easier to stop. In fact, it is the opposite. If you put the same pressure on the pedal, let's say 100 lbs of force, and have a leverage factor of 8:1 in the pedal then you are placing 800 lbs of force on the piston in the master cylinder. So to get pounds per square inch you'd divide by the surface area of the piston: 1.0625 diameter = .89 square inches of area = 899 psi 1.125 diameter = .99 square inches of area = 808 psi So a smaller piston actually gives you more pressure to the brake pads/shoes. However, if you go with too small of a piston you may not have enough volume to move the brake pistons as far as they need to go so your pedal would hit the floor before you had full braking. So check out what master cylinder you have. It may be embossed on the side of it like mine is. 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. Link to comment Share on other sites More sharing options...
grumpin Posted August 15 Share Posted August 15 Hmm, I'm going to take a look at the master tonight, but now I'm starting to think that it's the correct one for the truck. My truck was 460 powered originally. I did a little more research and it should be rated for over 8500 GVW, and it would have had the larger master and booster, right? It could have been changed somewhere down the line, but who knows? Now I'm wondering if my brake pads are glazed from trying to stop the truck with the blown rear wheel cylinders? I have a set of new pads on the shelf, I may replace them and see if it stops better. My 86 F250 had stiff brakes and my front calipers were rusted up and barely moving. Might check the calipers and wheel cylinders too. Link to comment Share on other sites More sharing options...
viven44 Posted August 16 Share Posted August 16 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. Here is a video of me testing it with a larger vacuum pump Link to comment Share on other sites More sharing options...
mat in tn Posted August 16 Share Posted August 16 Here is a video of me testing it with a larger vacuum pump well done on the video and showing the fail vs sound. Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 16 Share Posted August 16 Here is a video of me testing it with a larger vacuum pump I like it. But how are the brakes on the Bronco now with the new booster? Link to comment Share on other sites More sharing options...
viven44 Posted August 16 Share Posted August 16 I like it. But how are the brakes on the Bronco now with the new booster? 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. Link to comment Share on other sites More sharing options...
viven44 Posted August 16 Share Posted August 16 I like it. But how are the brakes on the Bronco now with the new booster? New booster did not make a world of difference.. if I thought it did it is probably placebo effect http://www.garysgaragemahal.com/uploads/6/5/8/7/65879365/zipper-23x23_orig.png 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. Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 16 Share Posted August 16 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. Well, at least you know you don't have a problem and you aren't sucking dirty air into your intake. Link to comment Share on other sites More sharing options...
viven44 Posted August 16 Share Posted August 16 Well, at least you know you don't have a problem and you aren't sucking dirty air into your intake. Yes very true on both! Link to comment Share on other sites More sharing options...
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