Testing Hydroboost - Rears Won't Stop With Panic Stop

[the-glove]

I'm trying to finish off a hydroboost conversion and was testing the brakes today, but rear wheels are not stopping when stomping on the pedal. I have not tested the fronts yet.

1995 Astro hydroboost (junkyard and I replaced the seals) and MC (new Raysbestos).
Mounted on an angle bracket
New Borgeson dual return pump
Proportioning Valve (combination valve that came with Tom's Broncos vacuum brake kit. Not adjustable)
Front discs/Rear drums

I've installed everything and wanted to test the brakes on jack stands today before taking it on a road test. Put the rear axle on stands, started the engine and tested if the brakes overcome the engine torque with tranny in Drive as well as if it could stop the wheels if I gassed it with the brakes on. It works ok on the first but when I gassed it a bit while holding down the brakes the wheels could still move. My initial impression is that, when stopping the wheels as they spin in Drive, I thought it would stop the rear wheels quicker and higher up on the pedal when on stands since it doesn't have to overcome the weight and momentum of the car. They do stop, but I have to press a bit deeper than I would have thought.

The big problem though, is if I try to simulate a panic stop by stomping on the brakes as the wheels spin....I get nothing! The pedal feels fine, but the wheels don't stop. I have to let the pedal up then push with more normal driving pressure and it will stop. If I stomp on it again, the same thing happens.

Any ideas??? Is it the proportioning/combo valve? I know that in a panic stop proportioning/combo valves are supposed to lessen the pressure going to the rear drums so that they don't lock up more than the fronts. But it doesn't even seem to be slowing it down at all when I simulate a panic stop. Do I need to go to an adjustable proportioning valve?

Also, should I be concerned that with normal pedal pressure it isn't stopping the wheels with more authority in the first inch or two of the pedal? I need to press a bit further to get it to stop quicker. I would think if I'm on the road and needing to stop the full weight of the car it's going to be even worse. Or is just because these are the rear brakes and most of the braking power is in the front?

I bled the power steering/hydroboost system until my shoulders hurt. I did:
- Pitman arm off steering box.
- Filled pump reservoir then did 40 cycles of left and right turns of steering wheel until no more bubbles came up in reservoir
- Cranked engine with fuel pump off three times
- Did maybe 90-100 cycles of turning steering wheel to full lock left and right 5 times each then pumping brakes 3 times for each cycle. I was still getting some slight fluid rise and fall in reservoir at the end, but not really any bubbles or burping....and these were if I was turning the wheel at a faster speed.
- Turned engine on, warmed up for 2 mins, and did a bunch of turns and pedal presses

I bench bled the master cylinder and rear drums. Used gravity method for the rears using a hose in a bottle. Let run until no bubbles. I had tried vaccum bleeder kit first but air was getting into bleeder threads, even when covered in grease. I have not bled the fronts yet.



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[DirtDonk]

I can't say for sure, but that sounds like a hydraulic (as in brake fluid movement) issue rather than a lack of power assist from the booster. You just can't tell that from an in-the-air test I would not think.
But yes, it should stop the rear tires dead as soon as fluid starts to open up the shoes. So the excessive pedal travel issue (though we can't tell from here if it's actually excessive or not until you measure how far the pedal moves) would seem to be an adjustment thing.

How much gap is there between the booster rod and the back of the master cylinder piston? New master was the same application as the booster? Did you verify gap anyway? With a roughly 6:1 pedal ratio, an additional 1/8" gap at the booster rod equals 3/4" pedal travel before anything meaningful happens.

What about the rear brake adjustment? Are the brake shoes up against the drum pretty good so that when you rotate the tire by hand you hear a decent amount of scuffing sound? To the point it puts just a little bit of resistance to your spinning?
With a limited slip or locker this is hard to tell, but if you have an open diff it's easy to determine if the brakes are starting to drag or not.

If the pedal is solid when it stops, it seems like all the air is out and your issue would be some adjustment somewhere instead.
If the pedal still feels a little soft when it's bottoming out, you have air (or component flex) still.
Yes, it could be the new proportioning valve limiting pressure too much. These type are well known for being defective in some manner or another when new. Most of the issues involve leakage around the delay valve, but I've had them block flow to the front brakes before. No reason to think they could not cause a problem with the rears just as easily.

But you'd have to exhaust a few more checks before consulting Tom's about a replacement unfortunately. Still a few other things it could be.

Paul

 

[nvrstuk]

I think Paul hit the nail on the head... but first. What size rear drums- stock? Do you have a proportioning valve still?

How did you adjust the rear brakes. I'm guessing the drums aren't adjusted properly. How did you do that?

 

[the-glove]

I can't say for sure, but that sounds like a hydraulic (as in brake fluid movement) issue rather than a lack of power assist from the booster. You just can't tell that from an in-the-air test I would not think.
But yes, it should stop the rear tires dead as soon as fluid starts to open up the shoes. So the excessive pedal travel issue (though we can't tell from here if it's actually excessive or not until you measure how far the pedal moves) would seem to be an adjustment thing.

How much gap is there between the booster rod and the back of the master cylinder piston? New master was the same application as the booster? Did you verify gap anyway? With a roughly 6:1 pedal ratio, an additional 1/8" gap at the booster rod equals 3/4" pedal travel before anything meaningful happens.

What about the rear brake adjustment? Are the brake shoes up against the drum pretty good so that when you rotate the tire by hand you hear a decent amount of scuffing sound? To the point it puts just a little bit of resistance to your spinning?
With a limited slip or locker this is hard to tell, but if you have an open diff it's easy to determine if the brakes are starting to drag or not.

If the pedal is solid when it stops, it seems like all the air is out and your issue would be some adjustment somewhere instead.
If the pedal still feels a little soft when it's bottoming out, you have air (or component flex) still.
Yes, it could be the new proportioning valve limiting pressure too much. These type are well known for being defective in some manner or another when new. Most of the issues involve leakage around the delay valve, but I've had them block flow to the front brakes before. No reason to think they could not cause a problem with the rears just as easily.

But you'd have to exhaust a few more checks before consulting Tom's about a replacement unfortunately. Still a few other things it could be.

Paul

Thanks for the reply! It’s an Astrovan hydroboost as well as master cylinder so they should match. The gap between the two is very slight....not even measurable in inches.

I haven’t tried adjusting the back brakes yet. But if there is too much gap between the pads and drums, would there be a difference between normal driving brake pedal pressure and a panic stop style pressure? Wouldn’t it then also not work when I put normal pressure on it?

I just went and bled the front brakes and then tried jacking up the rears again to see if it made a difference. And it was actually working with the panic stop pushes too now. But then I tried putting my left foot on the brake and pushed on the accelerator with the right to see if it had enough pressure to hold the wheels back. It did to an extent but the wheel would move a little if I gave it enough gas. But then after I did this a couple times, it went back to the wheels not stopping with a panic stop stomp on the pedal. I tried just normal pressure for a minute, and then it seemed to start working again on the panic stop stomps. It’s all just sort of inconsistent.

I’m thinking the proportioning valve has partly shut off pressure to the rears from the changes of front and rear pressure while I’ve been monkeying around with the bleeding, and needs to be re-centered. Does that make sense? When valve gets tripped like that, is it possible that it only partly shuts down pressure to the rears? Looking around online there seems to be varying results on how and the effectiveness of trying to reset the valve.

 

[the-glove]

I think Paul hit the nail on the head... but first. What size rear drums- stock? Do you have a proportioning valve still?

How did you adjust the rear brakes. I'm guessing the drums aren't adjusted properly. How did you do that?

I’m not sure on the size of the drums. It’s a full width big bearing 9 inch off a 1972 F100.

Yeah I’m running a proportioning valve that came with the Tom’s Broncos vacuum assist kit that I bought and installed a few years ago.

I haven’t tried adjusting the rear brakes yet. But if they need it, wouldn’t that also make it not have brakes with the normal driving pressure I tried?. Thanks.

 

[68rustbucket]

Maybe you need an adjustable proportioning valve.

 

[DirtDonk]

Even though I've seen it before, I can't actually explain the inconsistencies to the level that it would point to one thing wrong.
I had brakes once that seemed to work almost normally, but were a bit weak. If you hit the brake pedal suddenly though, the rears would lock up at the drop of a hat even if you were not pushing that hard.
In other words, it was similar to a panic stop, but without actually pressing hard like in a panic stop. But they still locked up every time.
Wasn't my car (until recently) and we never figured it out. Maybe I will someday, but for not it's not a driver anyway, so down on the list of things-to-do.

Oh, and four wheel drums, no prop valve, and just a single reservoir master. So literally nothing in common.

Paul

 

[gnpenning]

I’m not sure on the size of the drums. It’s a full width big bearing 9 inch off a 1972 F100.

Yeah I’m running a proportioning valve that came with the Tom’s Broncos vacuum assist kit that I bought and installed a few years ago.

I haven’t tried adjusting the rear brakes yet. But if they need it, wouldn’t that also make it not have brakes with the normal driving pressure I tried?. Thanks.

You need to adjust your brakes before bleeding or adjusting pedal travel. Have you checked the shoes yet? Nothing else can be determined for sure until you do these things first. This is step one.

 

[71 CA Bronco]

You need to adjust your brakes before bleeding or adjusting pedal travel. Have you checked the shoes yet? Nothing else can be determined for sure until you do these things first. This is step one.

Times 2. I would also get rid of the proportioning valve all together. You need most of your braking power to the front brakes.

Upon a recommendation from someone on this board. I plumbed my hydroboost directly to the front with nothing and rear with a Wilwood adjustible proportining valve (Wild Horses). It works awesome. I'm 4 wheel disk though.

 

[the-glove]

You need to adjust your brakes before bleeding or adjusting pedal travel. Have you checked the shoes yet? Nothing else can be determined for sure until you do these things first. This is step one.

Ok so check shoes and adjust to make sure they’re slightly dragging on the drums like Paul said above?

 

[gnpenning]

Ok so check shoes and adjust to make sure they’re slightly dragging on the drums like Paul said above?

Correct.

I like to tighten until you can't turn the drum then back off until you get a slight drag

 

[the-glove]

I adjusted the rear shoes and was able to get consistent stopping of the rear wheels with panic stops on the jack stands. Went out for a test drive and there's plenty of stopping power! It locks up too easily than I would want with hard braking, but feels good with normal traffic driving. With 30-40 mph stops, the fronts lock up about a second before the rears. I think less of a gap is better but it's probably close enough.

Thanks everyone for all your help!

 

[the-glove]

The tabs that Catfan referred to that weld onto the angle bracket. Also picture of Catfan’s bell crank with the Tom’s Broncos bracket.