How Much Vacuum Should a Good Brake Booster Hold?

[Gary Lewis]

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

There is a difference somewhere in the F100, F150, F250, F350, F450 range on the stud spacing. I'll have to dredge up that info if someone else doesn't come up with it.

But for now can you email me the file? I can put it up with the caveat that we are researching the width issue.

[viven44]

There is a difference somewhere in the F100, F150, F250, F350, F450 range on the stud spacing. I'll have to dredge up that info if someone else doesn't come up with it.

But for now can you email me the file? I can put it up with the caveat that we are researching the width issue.

Just emailed you

[Gary Lewis]

Just emailed you

Got the files but emailed back asking for a bit of explanation.

[1985 Bronco]

Got the files but emailed back asking for a bit of explanation.

Update with my problem: I had a mechanic friend drive the truck, and he recommended replacing the master cylinder. I've never dealt with a failing one before, and I guess I didn't realize that it was bad.

Replaced that, and the truck stops must better now. It still seems a bit soft to me, but maybe that's just 1 ton brakes trying to stop a big heavy truck? Never owned one before.

I do hear a slight hissing noise when I step on the pedal, so I'm still thinking about replacing the booster. I also still have those new front pads I could install.

[Gary Lewis]

Update with my problem: I had a mechanic friend drive the truck, and he recommended replacing the master cylinder. I've never dealt with a failing one before, and I guess I didn't realize that it was bad.

Replaced that, and the truck stops must better now. It still seems a bit soft to me, but maybe that's just 1 ton brakes trying to stop a big heavy truck? Never owned one before.

I do hear a slight hissing noise when I step on the pedal, so I'm still thinking about replacing the booster. I also still have those new front pads I could install.

Good update. Apparently the master wasn’t operating both sets of brakes, so it is good you replaced it.

And many times a failing master will put brake fluid into the booster, causing it to fail. But a hiss is a sign of a failing booster, so it should be replaced anyway.

You should have better brakes after that, but the next step after that might be the new pads.

[viven44]

Good update. Apparently the master wasn’t operating both sets of brakes, so it is good you replaced it.

And many times a failing master will put brake fluid into the booster, causing it to fail. But a hiss is a sign of a failing booster, so it should be replaced anyway.

You should have better brakes after that, but the next step after that might be the new pads.

How about the rear brake self adjustment ?

If I installed it with a lot of slack, will the routine reverse/hardbrake procedure fix that or will I need to install it right the first time (basically just tight enough) for the self-adjustment to work correctly?

[1985 Bronco]

How about the rear brake self adjustment ?

If I installed it with a lot of slack, will the routine reverse/hardbrake procedure fix that or will I need to install it right the first time (basically just tight enough) for the self-adjustment to work correctly?

I just replaced the booster, and it still doesn't stop all that well. It does stop, but if I had to do a panic stop I don't think I would be able to.

Even with the new booster it still makes the squishing noise when I step on the pedal. The pedal seems hard and it feels the same all the way down in the stroke. I'm going to pull off the rear drums and double check the adjustment, but I thought I had them adjusted well when I rebuilt the brakes.

Maybe I still have air in the lines somewhere? Front brakes hoses have a date of 1999 on them, but a bad hoses don't seem to match my problem. Any other suggestions?

[1985 Bronco]

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.

Well, both rear wheel seals are blown. Everything is covered in gear oil. I guess that explains why the brakes don't work very well.

I'm sure the rear brakes were doing absolutely nothing, and just the fronts were stopping me.

 

[viven44]

Well, both rear wheel seals are blown. Everything is covered in gear oil. I guess that explains why the brakes don't work very well.

I'm sure the rear brakes were doing absolutely nothing, and just the fronts were stopping me.

 

- Are all 4 brakes gravity bleeding ? In my experience lack of gravity bleeding equals poor braking and increased braking effort.

- About 2 days ago I realized the front brakes on my bronco were not gravity bleeding. Turns out with the engine not running, and the front wheels in the air we could not get the brakes to apply enough pressure to stop my hand from spinning the tires

- My wife and I could not get the front brakes to bleed at all without the engine running. What I mean by that is the brake pedal would not go to the floor even with the bleeder screw open, so we had the engine running and then we were able to get the front brakes to bleed. A lot of air came out both sides.

- now my front brakes are gravity bleeding. The bronco isn’t any sports car now but the brakes are a bit more responsive now. I think I can improve it further by going to different brake pads in the front. I am currently using RAYBESTOS MGD50M R-Line which were literally $10 for the set. I guess I can't expect a ton out of these... they are semi-metallic so supposed to be good... just not great.

 

[1985 Bronco]

- Are all 4 brakes gravity bleeding ? In my experience lack of gravity bleeding equals poor braking and increased braking effort.

- About 2 days ago I realized the front brakes on my bronco were not gravity bleeding. Turns out with the engine not running, and the front wheels in the air we could not get the brakes to apply enough pressure to stop my hand from spinning the tires

- My wife and I could not get the front brakes to bleed at all without the engine running. What I mean by that is the brake pedal would not go to the floor even with the bleeder screw open, so we had the engine running and then we were able to get the front brakes to bleed. A lot of air came out both sides.

- now my front brakes are gravity bleeding. The bronco isn’t any sports car now but the brakes are a bit more responsive now. I think I can improve it further by going to different brake pads in the front. I am currently using RAYBESTOS MGD50M R-Line which were literally $10 for the set. I guess I can't expect a ton out of these... they are semi-metallic so supposed to be good... just not great.

Yes, they bled just fine. I suppose if they are bleeding fine, then the rubber hoses are probably still good.

I think the rear brakes were just doing nothing, they're covered in gear oil. I started replacing the wheel seals tonight.

Hoping to drive the truck out of town on friday. I need to get this braking issue solved and cut a hole in the floor and mount the transfer case shifter. And get some test driving in before leaving.