With the key off, pump the pedal a few times to discharge the booster, then hold the pedal for a minute - does the pedal sink any further? If not, it’s not air.

If it does, Here’s a couple of other things to try that may help remove any more air.

1) Remove the front tires and MC reservoir cap. Remove some fluid from the reservoir to prevent overflowing, then use a C clamp to retract the calipers.

Then press the brake pedal until the pads contact the rotors and the pedal stiffens up again.

Repeat this 2 or 3 times as it can push air towards the reservoir and out of the system.

2) Get the front end up at least one foot - a pair of ramps or parking on a hill with the nose up are ideal - the point is to raise the front calipers above the level of the rear wheel cylinders.

Leave it parked for a day or two, then check pedal feel and/or drive it. Repeat until the pedal feels normal.

This isn’t much different than gravity bleeding and the theory is simple - air will always rise to the highest point in the system, so if the nose is up, air trapped in the rear circuit should work its way back to the reservoir. But it can take time, which is why leaving it parked nose up for several hours is necessary.

It may also help to rig the brake pedal to be depressed while doing this, as releasing the pedal creates a minor vacuum while the piston returns to its at rest position and any air that reaches the MC will be suctioned back into the reservoir.

You’ll have to either disconnect the battery or remove the brake light fuse to prevent the brake lights from draining the battery if you rig the pedal.

Good luck, and happy thanksgiving.

 

@Gumby22 Interesting. Very interesting. I will have to try some or both of those. And, also right back at you!🦃

 

I proped the break pedal down with moderate presure between the seat.

Here is the aproximate angle of the truck.

 

So, I found out something very interesting today. But first, I must summarize the results of what @Gumby22 recommended me do. I parked the truck on an angle, nose up, with the front of the truck probably about 8" - 10" higher than the rear, hopefully the picture with the level conveys the idea, then I propped the brake pedal down with a 2x4 as pictured and left if for I believe 2 days. I got spooky results from the test, but thankfully, I think the truck may be healing itself. This morning, after taking the 2x4 off of the pedal and lowering it back onto the ground, the truck did not want to come out of park. It took a minute to get it into gear, but I had to push down the brake pedal VERY far to do so. And this did not seem to make the pedal any firmer, but I think I know why.

2x4 propping brake pedal with moderate force. (No offence to Gumby, but I would not [size=13px]recommend[/size] [size=13px]propping[/size] the pedal down, as it messed with my shifter, [size=13px]making[/size] it difficult to get the truck out of park, hopefully it comes around)

Approximate level at which the truck was positioned.

I drove the truck over to my grandfather house, and we started to trouble shoot the truck a bit more. First we where very curious as to why the pedal needed to be pressed so far, but never could find why it was acting the way it was. He was looking for a cable or a linkage under the dash, from my understanding most vehicles have a cable that goes from the pedal to the shifter to only let you pull it out of park if you have your foot on the brake, he was not able to find that, so we think it is hydraulic. But that is not the point of this post, especially since the problem seems to be going away, I think have the pedal pushed down for that long did some funny business. And I must mention, their was still pressure on the board when I removed it, so their was not noticeable leak down in pressure, which I guess is good.

Where we found something interesting is when we started to diagnose the soft pedal again. We noticed that the pedal seems to pump up ok when the truck is off, but is very soft and spongey when the truck is running. First, we did a simple check to see if the pin in the booster was adjusted properly, and to verify that it was not the cause of the soft pedal. We found a pretty clever way of doing the test. What we did was loosen the two nuts that hold the master to the booster, and with one person slowly pushing the pedal, the other person would lean into the master, to hold it against the booster, and shout at the person pushing the pedal when the master started to move away from the front of the booster. We found with this test that when the pedal began to move down, that the master would almost immediately get the push. After testing that we began to questing the ABS again, like if it had power when the truck was on, and was doing something to make the pedal soft, we played around disconnecting the connector pictured below, and we also pulled the fuse. We noticed no change in the pedal doing any of this

We disconnected this connector from the ABS hydraulic unit. We also pulled the fuse. No results from this test though.

Fueled by curiosity, I wondered if the brake booster could have something to do with it. Thus far I have not really questioned it because, I would think, that a failed booster would only make the pedal harder, but a little testing, and research online says otherwise. We pulled the vacuum hose off of the booster, and plugged it with a ratchet extension (it is a pretty large hose, 1/2"), and fired the truck up, and even with the truck running, booster disabled, we had a hard pedal, and a almost rock hard pedal at that. The travel was almost nothing, pedal right at the top, not the mush that we would experience with it hooked up and the truck running. To me, this means I don't have any trapped air in the system, because to me, if their was air in their, it would be soft under all conditions. We did a small "test drive" if I can call it that up and down the gravel road, and the pedal remained good and hard, right at the top, and I could stop the truck with the manual brakes (though not without significant effort!).
So at this point, I am highly suspicious of the brake booster. We did a short test by putting a vacuum gauge in between the engine and the booster to see if their was a significant drop in vacuum when it was applied, and their was almost none, suspiciously little in fact. Where I am at we are at about 4200 feet above sea level, and at idle my truck produces about 15 inches of mercury, and hitting the brake pedal barely make the needle on the gauge quiver.
So, that is what I found. I no longer believe that I have trapped air in the system, but am left guessing on how to do a little diagnosis on this booster, and thus far, I have not found much on diagnosis on the internet, but I have found some pages that say a failing or failed booster can cause not only a hard, under boosted pedal (as I expected) but also it can cause a soft pedal, one that feels very similar to one with trapped air. Any advice on how to diagnose this booster is welcome, as I am wondering if the brake fluid leak from my original bad master cylinder got down inside the booster and ruined the rubber or something? Thanks for reading.

 

The confusion continues, sorry in advance @Gumby22 .
Today we replaced the booster in the truck, as I was having trust issues with it as noted in my last post. Replacing the booster went surprisingly well believe it or not, I had the old one out and the new one back in after only about 1 hour. Where it started to get weird was when we put the master back on, and started to bleed the truck. I should mention that after replacing the booster, we did check for the travel the pedal would move before it would contact the master cylinder, in the same manner I believe I mentioned in my previous post, and it checked out. But any ways, the truck was not acting right once we began the bleeding process, at first, we could get NO fluid out of the rear wheel cylinders. We could take the screws completely out, and slam or push on the pedal, and absolutely nothing would come out. The pedal would also not go all the way to the floor, maybe a 1/3 to 1/2 way down. The fronts where bleeding normally though, and when bleeding those the pedal would move in the normal way. We did eventually get fluid to start coming out of the rear bleeders, but not supper strong, also, we did get the pedal to start dropping after just keeping at bleeding it for a bit. It is very strange, if you repetitively bleed it, the pedal will not go to the floor once the bleeder is opened, in fact, you can not even feel when the bleeder is opened in the pedal, but if the pedal has not been pushed for a couple seconds, then you pump it up, the pedal will fall when the bleeder is opened. We ran out of time today to finish bleeding it, and make sure all the air was out, so that is something to consider. We plan on continuing the bleeding prosses on Sunday, to hopefully get the remaining air out, especially since we do believe the master ran dry one time when the truck sat with no bleeders in it (had to run to the parts store to get new bleeder screws because the old ones where so tired) we did not pump the pedal though when it was dry.
So does anyone have any thoughts? Is my proportioning valve going out? What could be going on?

 

Did you bench bleed the MC before installing it on the new booster?

 

@Gumby22 No, I did not, didn't fugure I needed to as it was full of fluid, and I had been driveing for a bit with it.

 

If you disconnected the hydraulic lines from the MC, it should be bench bled as it’s likely air has gotten into the piston bore. This would explain the lack of fluid flow to the rear brakes when attempting to bleed.

I’m still mulling over the rest of your posts. The winter cold is finally starting to set in, and battling my hibernation instincts drains most of my energy.

 

@Gumby22 Hehe! I get the feeling! The truck was bleed for another 2 hours today, so I belive all the air is gone. I have not driven the truck yet though.

 

So, its the next day, and I drove the truck, and belive it or not, the pedal is FINALY nice and stiff and up at the top. I don't know if it was replaceing the booster, or if it was that yesterday, the truck was bleed for 2 straight hours to get the air out from disconecting the master. In finall, I would not bother messing with the pintle again, as it never seemed to do anything, and same story with the ABS. I would get a gallon jug of brake fluid, and a few people to help bleed, and use the entire thing. I think these trucks like to trap air and or, the boosters fail in a way that makes the pedal soft. I belive I described it in anothe post, but I will again to be thourogh, but I was told that durring yesterdays marathon bleed, sometimes, after the pedal had been pumped up and then the bleeder was cracked open. The pedal would not drop at all. Sometimes it would go all the way to the floor, sometimes only half way down. I have no good clue as to what this could be, but I am not complaining, and I am not going to mess with it, the brakes work fantastic right now! I did a few VERY hard stops, and was abble to engage the ABS multiple times tonight on dry pavement. So, in conclusion, if you have to replace the master cylander, get yourself a booster while you are at the parts store, because as you can see, the first post I made, was OVER A MONTH AGO. I look at it as cheap insurance, especially as it was not too hard to replace, just make sure to get a new clip so you dont have to wory about reusing the old one. Their are plenty of good videos on how to replace the booster, but once you get the basic idea, it becomes self explanatory. It also might be a good time to replace your brake light switch too, since you will already have it out and it can cause your cruise control to not work (I replaced mine a year or 2 ago for that reason). I would not bother trying to do an ABS bleed, or use the previously mentioned pintle depression method until you have agressivly bleed the brakes. I hope this thred helps anyone who is haveing issues, and thanks to those who where very helpful, like @Gumby22 .