Brake Booster Losing Pressure with Engine Running

[JeepGuy]

SBC's do tend to want a little more initial than SBF's, although both like to have about the same total.

If they're the Chevy calipers then they're a 2-15/16" OD piston. Out of curiosity, have you tried pulling the booster check valve out of the booster and then trying the pedal that way? That would yield about an 82:1 total (hyd + pedal) leverage ratio, which is pretty stiff. For pure manual brakes you'd want it closer to 95:1. For vacuum boosted brakes operating correctly that's probably about the right ratio.

I think I missed part of this post. What do you mean by pulling the check valve out of the Booster? Are you talking about the valve the vacuum line hooks up to? If so, I have not tried that yet.

 

[ntsqd]

Yes, the vacuum check valve at the booster. Pulling the valve out of the grommet in the booster (not usually very easy to do) should zero out any effect that the booster has on how the brakes work and feel.

However, with the revelation that the pedal will slowly go to the floor I would be looking at the m/c as that sounds like a hyd. leak.

 

[JeepGuy]

Haha! Fortunately, Jon (admin) made it even easier than it was before, when he updated the software a couple of years ago.
All you need to do, after you hit the "reply" button to get the quotes, is put the cursor where you want to break a paragraph or subject, then when you hit ENTER (or RETURN in some cases?) it automatically separates the part you want to individually quote, from the rest.
So for this most recent reply, I put the cursor after the word "line?" in your question, then hit the enter button. Voila!

Uh oh... Leaned me something new today!

Yep, looks like pretty standard Bronco livery. Should be ok then, as long as the outer damper ring has not slipped.
But aside from slipping, it sounds like your visual test with the piston confirms that you're still there. Or at least close enough for now.

That's a very interesting observation! Glad you pointed it out, but I can't tell you from my own experience whether it's a good thing, or a bad thing.
Maybe others will have some input on the subject.

Did we already discuss whether or not you have verified your gear ratio? This could be very important to your performance concerns. Those 35's with any but the perfect gear ratios could be very sluggish in the acceleration department.
Especially if you still have 3.50's in the diff!

I have never actually checked to see what gear ratio I have. I believe they are 3:50's but they could have been changed years ago before I purchased the Bronco.

I will say, it is a bit sporty with the current set up (302 unk build, C4, Dana 20). The other day I actually beat a Prius off the line. I'm not sure they knew we were racing, but I'm taking the win regardless!

Need to verify two things.
Get your idle where it should be, but a little higher for good vaccum is not the end of the world.
But a slowly descending pedal usually means that there is a leak somewhere in the system. Usually in the master cylinder, but not exclusively to that. Or at least that's my take on it.

Now that I have some idea of what the timing is actually set at, I'll go out and set it strictly according to the Vacuum gauge at the high end and go drive around a bit. As it is now the needle at idle sits at the low end of the green section (normal vacuum) section of the gauge for whatever that is worth.

 

[JeepGuy]

Yes, the vacuum check valve at the booster. Pulling the valve out of the grommet in the booster (not usually very easy to do) should zero out any effect that the booster has on how the brakes work and feel.

However, with the revelation that the pedal will slowly go to the floor I would be looking at the m/c as that sounds like a hyd. leak.

I blocked off both ports on the MC and checked that a couple weeks ago. It held strong, but it wouldn't hurt to do it again. Based on the recommendations in here I had planned on separating the MC and Booster again to re check the rod length. I'll look closely for any moisture between the two. Based on my past experience, since it isn't leaking anywhere visible yet, if I do this again it for sure will leak.

I'm starting to think engine RPM and honestly Carb adjustment is a bit at play here. in relation to keeping the vacuum pressure in line. But that is just a guess. I rebuilt the Carb on the bench at home and just set the idle/fuel mixture screws based off YouTube university standards.

 

[JeepGuy]

Here is my simple, tool-less method of checking the gap.
Remove the two nuts holding the master cylinder to the booster.
Pull the master slightly away from the booster.
Adjust the rod out something minimal. Say one turn.
Gently push the master back in place and see if it bottoms out on the rod. If the rod touches down before the master cylinder is seated, turn the rod in half a turn and re-check.
If, after turning the rod out one turn, the master still does not touch down on the rod, repeat, and adjust out one more turn.
Repeat this until you actually can detect the master cylinder bottoming out on the rod, before it bottoms out on the mating surface of the booster.
When you get to this point, turn the rod back in half a turn, or until it no longer touches the piston first.

Someone here will probably remember, or you can look it up in the book, to see what the actual gap specification is supposed to be. It’s pretty minimal though, so keep that in mind.
When you’re satisfied that the booster rod is not touching the piston, but is just barely clear of it, lock things back down and take it for a test drive.

This has been pretty precise over the years for me. It is certainly not “by the book“ or kosher from a mechanic’s standpoint. But it has never let me down.

I have time today so I'll be doing this again. It will be about the fourth time, but there is always the possibility I'm missing something here.

Based on my prior checks, the rod depth as measured should almost bottom out against the MC when mated together. What I'm not positive about is the depth of the back of the MC versus how far it inserts into the Booster. That could account for a bit of a difference, but I do remember checking that and the two were very close.

I also recall that the rod from the Booster was almost at the end of its threads so I really don't think I have more than maybe a turn to adjust it out before it will fall out of the Booster.

 

[ntsqd]

If you suspect that there is a gap between the the booster push-rod and the m/c piston then an easy way to not only check it, but also measure it is to borrow the piston to valve clearance checking trick and put some modeling clay or Playdoh in there and reassemble it then pull it back apart. Slice the clay on the centerline and remove half of it.

 

[DirtDonk]

I remember a friend once had to create his own "bullet" spacer for the back of a deep-well master cylinder. Can't remember what booster he was trying to mate up to his master, but the rod and piston were obviously not compatible with each other right from the get-go.
Luckily he's a metal bits hoarder, and we found a perfect candidate (even had a recessed dimple in one end!) in his stash and made it all work together.

You'll find that, whether youtube-U or the factory books or your best friend's grandfather's advice, the adjustments to a carburetor are only valid once it's on the engine and running. All fine-tuning has to happen on the engine in question.
All other adjustments are baselines to get you running. After that, all sorts of tweaks are in your future.
Even the vacuum gauge is not perfect. Great for idle especially, but then you have to use your ear to see if the high vacuum setting is too high for your particular setup.

I forget, did you say you replaced the camshaft?

Paul

 

[JeepGuy]

I remember a friend once had to create his own "bullet" spacer for the back of a deep-well master cylinder. Can't remember what booster he was trying to mate up to his master, but the rod and piston were obviously not compatible with each other right from the get-go.
Luckily he's a metal bits hoarder, and we found a perfect candidate (even had a recessed dimple in one end!) in his stash and made it all work together.

You'll find that, whether youtube-U or the factory books or your best friend's grandfather's advice, the adjustments to a carburetor are only valid once it's on the engine and running. All fine-tuning has to happen on the engine in question.
All other adjustments are baselines to get you running. After that, all sorts of tweaks are in your future.
Even the vacuum gauge is not perfect. Great for idle especially, but then you have to use your ear to see if the high vacuum setting is too high for your particular setup.

I forget, did you say you replaced the camshaft?

Paul

I have not touched the engine other than to inspect and replace spark plugs and thermostat. Of course, I added power steering and replace belts and hoses etc... but nothing internal.

Plans for the future are ProFlo 4 which will eliminate some of my current tuning and carb related issues, but I'm sure will create other fun things to tweak here and there.

 

[JeepGuy]

Update:

So most importantly, Still no visible Fluid leads anywhere.... at least on the Bronco (more on that later).

So, I hooked the Vacuum gauge back up to the booster line and fired up the Bronco. With the engine warm, at idle, it is measuring just under 18 on the gauge. BUT... as soon as I put the C4 transmission in Forward or Reverse, the vacuum drops to 15. My engine RPM does drop significantly in gear vs neutral which I expect, but I tend to think this portion of the issue is leaning to a proper carb tuning.

On to the Booster pushrod. So I separated the Booster and MC again and both mating surfaces at completely dry. NO LEAK... good stuff. I started by measuring the distance of actual pedal movement by getting a starting point on the booster rod and pushing the brake all the way down with the MC separated. As best as I could measure it moves about 1 3/16's to 1 1/4. The bore on the MC is 1 1/8 from what I have read so that seems proper.

On to the pushrod adjustment. I confirmed the adjustable part of the pushrod was about a half thread from wobbly on there and I could not adjust it any further out. Quick trip to the local ACE Hardware and picked up some #8 all thread and a cap nut. Cut the all thread to length and ground down the cap nut hard line edges so it was all round. Compared that to the MC Bullet that was provided for different set ups to make sure I wasn't putting something in there that would bind. All is good.

Of course during that process a full bottle of Brake fluid decided to vibrate off the work bench leaving me with memories of the event for ever more.

Lesson learned here... Either the Brake Fluid or Brake Cleaner did a bang up job of removing the film from the BFG logo... nice and shiny white now.

Anyways, a little filing of the thread and in went the new pushrod end. I followed the technique laid out by DirtDonk. The rod is as close as I could get it using 1/4 turns of the rod to shorten it. As soon as the MC was flush I stopped adjusting in. In the end, there is a noticeable difference in the final setting and where the old rod ended up. Between gap and the wiggle in the pedal, I immediately noticed a difference in the pedal engagement.

I'll road test it a bit later... Currently working on adding more horse power to the dash .

 

[DirtDonk]

Very clever!
So basically, if you had turned the original one all the way in, and inserted the bullet into the master, it would’ve been too long to let the master seat all the way down?
Sounds like this was the only solution then.

The problem has been for decades, that everybody seems to make the boosters with different length rods. Some long, some short. Some adjustable, some not.
But usually there’s a happy medium, and they can still be made to work with whatever master you have.
Master cylinders differ as well, but I don’t think by as much.

Now we can’t wait to hear how it feels when it’s driving!

 

[JeepGuy]

Very clever!
So basically, if you had turned the original one all the way in, and inserted the bullet into the master, it would’ve been too long to let the master seat all the way down?
Sounds like this was the only solution then.

Yeah, it looks like if I tried to use the supplied bullet it would have been just a bit to much and I could have had pressure on the plunger at all times. I spoke with WH about that bullet and they basically said I wouldn't use it unless I was running hydro so that makes sense. From what I can tell there was at least an 1/8th if not bigger gap due to the comparison picture above with the plunger set in the Booster and the pin at about where it sat at full adjustable (right at wobble loose point).

The problem has been for decades, that everybody seems to make the boosters with different length rods. Some long, some short. Some adjustable, some not.
But usually there’s a happy medium, and they can still be made to work with whatever master you have.
Master cylinders differ as well, but I don’t think by as much.

Now we can’t wait to hear how it feels when it’s driving!

Road test this morning and the pedal feels much better with a little RPM. Brakes kick in almost immediately and there is a definite improvement in stopping power. I'm thinking the combo of the vacuum adjustment with timing and the properly adjusted plunger made that difference. Only thing now is I kinda wish I tested between both reworks so we could pinpoint the issue.

I guess in theory here it is possible due to the gap in the plunger, and limit on the stroke of the pedal, I never got to full insert depth during the MC Bleed on the truck. I guess that is a benefit of a bench bleed . Based on that possibility, I'm going to do another round of brake bleeding.

Also, now that I'm getting full plunge, I'm going to see if there is enough of the pin on the front of the proportioning valve sticking out so I can grab ahold of it during the bleed. Up until now, it only moved about 1/16th and there was no way I could grab it with anything. My only concern is putting a little burr on the pin restricting it from going all the way back in. We'll see, as the saying goes here, "If it ain't broke yet, keep fixing it until it is!"

 

[DirtDonk]

That's more like a mantra in my garage!

Paul