Help Diagnosing Bad Brake Booster

jesseshoots

jesseshoots

TJ Enthusiast
Joined
Feb 20, 2019
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Location
Charlotte, NC
So I'm trying to figure out what my brake issue is and I believe I've got it down to a failing booster. Calling the brake god @mrblaine

Background:
'97, 4.0, non-ABS
I clipped the left front hard line on the frame while off-road, lost brakes, and got it home and parked it. I swapped out the front lines with 3/16" stainless braided lines. I used one to run out of the distribution block to a tee fitting, then another line to each caliper from the tee. I converted to -3AN at the block and plugged the rear most hole. I used -3AN all the way thru until the banjo fitting at the calipers. I bled the brakes and got it all good... or so I thought.

Problem:
When I hit the brakes it is extremely hard at first, then once I lay into them it softens up. When I release the pedal the calipers stick for about three seconds. The only time I've ever not had this issue was when I got on it and revved it past 4k. After that the brakes operated normally for that stop. I've been able to replicate this.

Thoughts:
I believe the booster has a vacuum leak that isn't allowing the correct differential pressure when at anything under 4k rpm. How should I test this? If the booster is truly the problem, any recommendations on which one to buy or if there is a way to rebuild it (long shot I know)?
 
jesseshoots said:
So I'm trying to figure out what my brake issue is and I believe I've got it down to a failing booster. Calling the brake god @mrblaine

Background:
'97, 4.0, non-ABS
I clipped the left front hard line on the frame while off-road, lost brakes, and got it home and parked it. I swapped out the front lines with 3/16" stainless braided lines. I used one to run out of the distribution block to a tee fitting, then another line to each caliper from the tee. I converted to -3AN at the block and plugged the rear most hole. I used -3AN all the way thru until the banjo fitting at the calipers. I bled the brakes and got it all good... or so I thought.

Problem:
When I hit the brakes it is extremely hard at first, then once I lay into them it softens up. When I release the pedal the calipers stick for about three seconds. The only time I've ever not had this issue was when I got on it and revved it past 4k. After that the brakes operated normally for that stop. I've been able to replicate this.

Thoughts:
I believe the booster has a vacuum leak that isn't allowing the correct differential pressure when at anything under 4k rpm. How should I test this? If the booster is truly the problem, any recommendations on which one to buy or if there is a way to rebuild it (long shot I know)?
Click to expand...
Need pictures of the new plumbing. If the issue was not present before, it seems very coincidental that it would start after the plumbing.
 
Vacuum should be highest at idle and therefore brake assist would be most noticeable at lower RPMs. Depending on how bad the brake booster is, you should have assist building over time but after pressing the brake pedal it would lose it quickly and return to a hard to press pedal.

What you're describing doesn't strike me as a bad brake booster, IMHO
 
mrblaine said:
Need pictures of the new plumbing. If the issue was not present before, it seems very coincidental that it would start after the plumbing.
Click to expand...
Agreed, I’ll snap some pics tomorrow. Don’t be too harsh on the routing as the lines are temporarily secured with zip ties until I get a chance to weld some studs to the tube.
 
The connection to the distribution block is a 3/8”-24 to -3AN adapter on the port with the F (front) and stock for the rear. I blocked the back port with a 3/8”-24 plug. This goes to a 3/16” stainless braided Fragola line that runs down the rear hard line, to the frame, and down the upper control arm to the -3 tee fitting in the second pic.

78A1AEB8-BFE4-4DC7-A6E1-90E60728261B.jpeg


Off the tee are the left and right 3/16” lines with banjo fittings on the opposite ends. The only “sketchy” part here is that I got my numbers wrong when I measured my lengths and ordered about 6” too long on each line. This led me to cross over the sides keeping in mind a large arc for fluid flow. I don’t know about hydraulic fluid, but in the air moving industry it’s a minimum bend radius of 2D for steady flow which I easily achieved here.

BE41E5AF-49A7-4444-AEAA-D723ACD4EF9D.jpeg


393254BD-79CE-4E50-9224-66AE20F5EB0D.jpeg
 
jesseshoots said:
The connection to the distribution block is a 3/8”-24 to -3AN adapter on the port with the F (front) and stock for the rear. I blocked the back port with a 3/8”-24 plug. This goes to a 3/16” stainless braided Fragola line that runs down the rear hard line, to the frame, and down the upper control arm to the -3 tee fitting in the second pic.

View attachment 197920

Off the tee are the left and right 3/16” lines with banjo fittings on the opposite ends. The only “sketchy” part here is that I got my numbers wrong when I measured my lengths and ordered about 6” too long on each line. This led me to cross over the sides keeping in mind a large arc for fluid flow. I don’t know about hydraulic fluid, but in the air moving industry it’s a minimum bend radius of 2D for steady flow which I easily achieved here.

View attachment 197924

View attachment 197925
Click to expand...
First, forget that air flow bullshit for brake fluid. You are only moving a small volume for each full pedal press and then the pistons retract and push the fluid back to where it was in the line. Secondly, if you were adhering minimum bend radii for flow reasons, you violated the crap out of that with the 90 Degree drilled fitting off of the combination block, the hard 90 drilled Tee, and then the banjo fittings. All of those are high resistance low flow fittings and have no business being in a high flow installation. However, for brakes, they are perfectly acceptable.

Next, did you pull the master and bench bleed it when you clipped the line that started this?
 
mrblaine said:
Next, did you pull the master and bench bleed it when you clipped the line that started this?
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I did not bench bleed the master. It still had fluid left in the reservoir so I assumed it would be okay. Probably a bad assumption I’m sure.

My original thoughts were that I had air in the system somehow or I blew out a seal in the master, but the more I started researching it the less likely it seemed due to the symptoms.
 
jesseshoots said:
I did not bench bleed the master. It still had fluid left in the reservoir so I assumed it would be okay. Probably a bad assumption I’m sure.

My original thoughts were that I had air in the system somehow or I blew out a seal in the master, but the more I started researching it the less likely it seemed due to the symptoms.
Click to expand...
Let's take that out of the equation to be sure and we'll keep going after that. How did you bleed the system after the lines were installed?
 
mrblaine said:
Let's take that out of the equation to be sure and we'll keep going after that. How did you bleed the system after the lines were installed?
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I’ll do that this weekend.

I bled the system at the calipers first using the pump/hold method because it was the only way to build initial pressure. After that (an hour or so later) I tried your method mentioned somewhere in @toximus build thread where you open the bleeder valve, energetically push the pedal to the floor, then shut the valve. There may have been too much air in the fluid already at that point to really help though. I was planning on getting one of the kits that you vacuum bleed thru the master but have hesitated to pull the plug.
 
jesseshoots said:
I’ll do that this weekend.

I bled the system at the calipers first using the pump/hold method because it was the only way to build initial pressure. After that (an hour or so later) I tried your method mentioned somewhere in @toximus build thread where you open the bleeder valve, energetically push the pedal to the floor, then shut the valve. There may have been too much air in the fluid already at that point to really help though. I was planning on getting one of the kits that you vacuum bleed thru the master but have hesitated to pull the plug.
Click to expand...
Bench bleed by plugging the ports and then using the plugs as bleed screws. Push in on the push rod with the plug cracked, close plug, let the push rod out. That generates a vacuum that gets the fluid into the bore quickly. When you have fluid at the closest port, do the further one. When both have fluid, stroke the push rod until it only goes in 1/8" or so. Done.
 
mrblaine said:
Bench bleed by plugging the ports and then using the plugs as bleed screws. Push in on the push rod with the plug cracked, close plug, let the push rod out. That generates a vacuum that gets the fluid into the bore quickly. When you have fluid at the closest port, do the further one. When both have fluid, stroke the push rod until it only goes in 1/8" or so. Done.
Click to expand...
Any particular trick for plugging the ports or just use a fitting with some capped clear tube?
 
I was able to mess around with the brakes yesterday. I tried to remove the master cylinder to bench bleed but the nuts were on there too tight and were too soft to make anything happen without an impact. This resulted in me bleeding the master while still on the vehicle. I cant guarantee I did it properly, but I was able to get fluid to come out of each port and create a vacuum on each. The brake pedal stopped after a short distance, but since I couldn't measure how far the throw was I can only assume it worked.

I finally bought the Motive Pro pressure bleeder and that was super helpful for bleeding... at least the rears. Here is where I ran into issues that I didn't notice before. I pressurized the reservoir to 15 psi, opened the bleed screw on the front right caliper, and only about an inch worth (in the clear line) of fluid came out of the caliper into the bleed bottle. Nothing more. The same thing happened on the front left. The only way to get fluid thru the calipers was to press the pedal down as hard as I could to get it to go to the floor.

My thoughts at the moment are that I need to take the lines and fittings off the front port of the distribution block and check each one individually for blockage.
 
Forgot to add that the pedal feel is about back to normal. The brakes still hold for a few seconds when I let off however. I'm in agreement that the booster is no longer the issue.
 
When you lost your front brake line, the shuttle valve should have moved to block the flow to the front brake lines...and you should have gotten a light on the dash. Did you? You might look at that thing and make sure it's centered and working properly.
 
Mike_H said:
When you lost your front brake line, the shuttle valve should have moved to block the flow to the front brake lines...and you should have gotten a light on the dash. Did you? You might look at that thing and make sure it's centered and working properly.
Click to expand...
I have a new MC and when I put it in I plan on taking the block apart and cleaning it.
 
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