OK another brake question

[ngsd]

I have built quite a few Broncos but these brakes are stumping me. Everything is new and I evern swapped out another new master cylinder but the brakes just keep going to the floor. 77 Brake booster and MC, Chevy front disc conversion with new calipers, rears are stock with new wheel cylinders, all new lines, brake proportioning valve. I have bled them to death, no air comes out. Bench bled the MC twice. I use a power booster and kept constant pressure on with no leaks. I am just asking to see if I am missin something. I had the same setup on it before but all the wear parts now are new.

 

[Beau Nugget]

The calipers aren’t upside down preventing the air from bleeding out of them, are they?

 

[ngsd]

No and also used a power bleeder.thanks

 

[ntsqd]

Have you verified the fit of the booster's push-rod to the m/c piston?

 

[ngsd]

As far as length? As I push on it the reservoirs are both active but not sure what other things to verify

 

[ntsqd]

This tool checks the length of the booster's push-rod against the the depth of the m/c's piston divot:

https://www.amazon.com/dp/B0BDDBNDBX/?tag=classicbroncos-20

It doesn't sound like this could be the cause of it being far off, but it's worth checking.

 

[Broncobowsher]

Rubber flex lines? Vice grip off front/rear and see if you can isolate it. If you pinch both rubber lines and the pedal is still going to the floor, you are dealing with an issue at the master. If the pedal firms up, what you have clamped off is why it is going soft.

I know you said you have the calipers on correctly. I don't believe you. Seen it too many times. Including pros that do this stuff for a living who spent over a day trying to fix it. Until there is a second set of eyeballs on those installed calipers I don't trust anyone who says they have them on right. That isn't just you, that is anyone.

As with any GM conversion on a Bronco, I also question caliper to knuckle clearance. New calipers may not have the clearance the old ones had.

 

[ngsd]

Rubber flex lines? Vice grip off front/rear and see if you can isolate it. If you pinch both rubber lines and the pedal is still going to the floor, you are dealing with an issue at the master. If the pedal firms up, what you have clamped off is why it is going soft.

I know you said you have the calipers on correctly. I don't believe you. Seen it too many times. Including pros that do this stuff for a living who spent over a day trying to fix it. Until there is a second set of eyeballs on those installed calipers I don't trust anyone who says they have them on right. That isn't just you, that is anyone.

As with any GM conversion on a Bronco, I also question caliper to knuckle clearance. New calipers may not have the clearance the old ones had.

Thanks, caliper bleeders are pointing up.

 

[ntsqd]

Mr. Bowsher brings up a point though, have someone simulate a panic stop while you're looking at the caliper to knuckle clearance. If that last calipers obtained clearance by being ground-on, rather than the knuckles as it should have been done so that this sort of problem does come up again, then the new calipers could be hitting the knuckles and that would give you the results that you have.

If the calipers are hitting the knuckles, then clearance the knuckles with enough room to allow for brake pad wear.

 

[ngsd]

Mr. Bowsher brings up a point though, have someone simulate a panic stop while you're looking at the caliper to knuckle clearance. If that last calipers obtained clearance by being ground-on, rather than the knuckles as it should have been done so that this sort of problem does come up again, then the new calipers could be hitting the knuckles and that would give you the results that you have.

If the calipers are hitting the knuckles, then clearance the knuckles with enough room to allow for brake pad wear.

It is the same brakes that came off of it. They worked great before.

 

[ntsqd]

In the first post you said that everything is new so I assumed that you had new or reman'd calipers. Specifically not the calipers from when it was working fine.

 

[ngsd]

In the first post you said that everything is new so I assumed that you had new or reman'd calipers. Specifically not the calipers from when it was working fine.

my mistake, yes the calipers have been replaced but only after I could not get them to work either. Same issue with old calipers. Thanks

 

[ntsqd]

Then the clearance to the knuckles should be checked, if only to confirm that it is still there.

When pushed to the floor does the pedal have any resistance at all?

 

[ksagis]

Long shot on this, but you sure the replacement MC has the same bore as MC pulled off? Reading thread again, I think the problem was there prior to MC swap.

Not totally clear if MC was changed as a result of troubleshooting issue or at start of the work.

Edit: were the brakes working okay, and after swapping wear items, pedal went away?

 

[ngsd]

I am not sure about that. I would think that even with a smaller bore I would get some pedal though. The proportining valve may be my next look as it was not the same one. MC was changed as it is a total rebuild and replaced everything with new as part of the restoration. Thanks

 

[ksagis]

I am not sure about that. I would think that even with a smaller bore I would get some pedal though. The proportining valve may be my next look as it was not the same one. MC was changed as it is a total rebuild and replaced everything with new as part of the restoration. Thanks

I think you’re saying you don’t know how brakes worked before, is that right?

If that’s the case, maybe the MC was undersized before and had bad travel.

One thing that I ran across in my way too deep dive into brakes is the D52 caliper swap to a Bronco doesn’t honor the bleeder port location, particularly with a rotated pinion geometry. Some folks have commented pressure bleeding should overcome this, and I agree to a point, but my pinion is rotated quite a bit due to suspension lift to maintain good driveshaft angles

 

[ksagis]

Pics per above post, you can make out the bleeder at nearly TDC if you zoom in and squint.

 

[ngsd]

I think you’re saying you don’t know how brakes worked before, is that right?

If that’s the case, maybe the MC was undersized before and had bad travel.

One thing that I ran across in my way too deep dive into brakes is the D52 caliper swap to a Bronco doesn’t honor the bleeder port location, particularly with a rotated pinion geometry. Some folks have commented pressure bleeding should overcome this, and I agree to a point, but my pinion is rotated quite a bit due to suspension lift to maintain good driveshaft angles

Sorry, to clarify the brakes were working fine before. The travel is one of the things I will check next along with the proportioning valve.

 

[ksagis]

Checking in, any luck or updates on this?

 

[ngsd]

No I went to San diego for a bit and will update when I get back to it next week.