Power Disc Brakes not working on hard stop?

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Dartman61 said:
OK based on discussion and your success without power brakes, I've, disconnected the brake pushrod - Pedal to Booster and travel of pedal is much greater than when connected - so not pedal. So given everything else the limit has to be between power booster and MC. Life is short and I need to resolve this - I ordered a new booster set up from Summit so I'm hoping that eliminates any issue between input to booster to output of MC. Will let folks know result of this experiment. Unfortunately I live in Minnesota and will have to wait until spring to really test but I'm hoping to get a different pedal feel. Will advise!

Sorry, no the brake pads are not glazed - apologize I didn't;'t mean to blow past it
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What kind of pads do you have ?
 
Well not knowing what you have isn't helping.
You could have some s*** organic pads on there, which isn't cutting it. Semi-Metalic would be my choice for best performance. This could be as easy as brake pads.

I would also suggest the brake forum, and do a search. Years of info there.
 
OK Sorry, what I meant to say is it would be more helpful for you to recommend a pad type than ask me what I have. I set up the system as stock so have nothing special. I'm willing to spend a couple bucks based on a recommendation. I'm retired, bored and would be more than happy to swap parts based on a recommendation that might improve what I have . Thanks, Steve
 
Dartman61 said:
OK Sorry, what I meant to say is it would be more helpful for you to recommend a pad type than ask me what I have. I set up the system as stock so have nothing special. I'm willing to spend a couple bucks based on a recommendation. I'm retired, bored and would be more than happy to swap parts based on a recommendation that might improve what I have . Thanks, Steve
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A good semi-metalic pad is my recommendation.
I'm looking for a new set as well. Having said that, it looks like semi-metalic pads for the Kelsey Hayes brakes are starting to dry up. Great eh ? Again, tons of reading in the brake forum.
Hope this helps.
 
For a streeter, I use organic cuz my rotors hardly wear. I got 125000 miles and counting on mine. Those 1970 factory rotors should be good for another 20 years
but, I admit that with 295s on the back and no proportioning, the rear shoes do most of the braking. And yes, I can, lock up the fronts when I need to, but you know, with an 11/1 367 and and a manual trans I also have plenty of compression braking.
 
Don't know how many times I have to say it: brake booster [ BB]. Based on the symptoms described in post #1.
Booster operation is quite complicated. Modern boosters are vacuum suspended, ie at rest both sides of the diaphragm inside are balanced by engine vacuum. When the pedal is moved, atmospheric pressure [ AP ] is admitted to the rear portion of the BB, & this force pushes on the diaphragm & BB pushrod, into the m/cl. The amount of BB assist is governed by valving inside the BB that regulates the amount of air entering the rear chamber. It should be obvious that if this regulating/reaction mechanism fails or becomes faulty.......you get the exact symptoms the OP is getting.
 
OP
Another new thread picture makes me want to ask if there could be any kind of interference with that gizmo that bolts on.
Seems to me, years ago, someone required a notch or somesort to clear the firewall, or linkage binding on something.
Just really scrutinize the linkage that appears to be hard to see.
Just throwing spaghetti. .
 
Bewy said:
Don't know how many times I have to say it: brake booster [ BB]. Based on the symptoms described in post #1.
Booster operation is quite complicated. Modern boosters are vacuum suspended, ie at rest both sides of the diaphragm inside are balanced by engine vacuum. When the pedal is moved, atmospheric pressure [ AP ] is admitted to the rear portion of the BB, & this force pushes on the diaphragm & BB pushrod, into the m/cl. The amount of BB assist is governed by valving inside the BB that regulates the amount of air entering the rear chamber. It should be obvious that if this regulating/reaction mechanism fails or becomes faulty.......you get the exact symptoms the OP is getting.
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It's a lack of travel Bewy,

But I do know the feeling, personally a few times . lol

horshack from kotter - Search Videos

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My comments assume the brake booster is correct for the application. The OP states that the pedal travel is normal 1" or so [ post #3 ] & pedal is 'rock hard '. That tells you: no air in the system.

So how can it be lack of travel??? Using that 'logic', you would not be able to bleed the brakes using the pedal because the you would not be able to move the pistons in the m/c far enough to push the air out!!
 
Bewy said:
My comments assume the brake booster is correct for the application. The OP states that the pedal travel is normal 1" or so [ post #3 ] & pedal is 'rock hard '. That tells you: no air in the system.

So how can it be lack of travel??? Using that 'logic', you would not be able to bleed the brakes using the pedal because the you would not be able to move the pistons in the m/c far enough to push the air out!!
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I think all those questions are answered in previous posts.
He has a vacuum bleeder, they work great .

https://www.summitracing.com/parts/...LnJmiXv4B2lrAnXa7sK-4iLN4Y6-TNz4aApQHEALw_wcB
 
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maybe i missed it, but can you provide some clarity: do the front wheels lock up? if not, can you get them to lock up if you work the pedal?
 
OK quick up date, I purchased a new brake booster from Summit RSD- RPB8001, believe its same one classic industries sell - look identical. Anyway I installed and now problem is worse. I have a rock hard pedal with no assist. I check vacuum and power booster canister holds 21 PSI vacuum for 20 minutes - Summit's test for good booster. With the old booster I had power assist but ran into rock hard pedal and limited braking. Braking feels the same without power assist now. $225 down the drain and worse off.
 
I wonder if this is all about the booster / master cylinder pushrod adjustment?
 
When I installed I made sure the booster to MC pushrod was set up per the gauge I mentioned earlier in the thread. I will play with it a bot tomorrow but I went from brake assist to absolutely no assist - so not thinking pushrod is the issue - its a new problem as I had assist with the old booster
 
I wonder if the gauge is the wrong one? Just seems like that it involves the interface between the booster and the master cylinder.
 
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In the pic from the Right Stuff site, the pic show slots on the firewall mounting brackets.
Now there is NO motion or assist, is it possible that by sliding the booster up or down can be causing the linkages to hit the firewall, or fender.
Perhaps the slots are what I remembered from the previous post I mentioned.


IMG_20240118_180548.jpg
 
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Summit sent you a bad booster...or check valve is defective.
 
OK - latest update. I have a vacuum pump made for power brakes on engines with low vacuum and hooked it up to my power brake booster - like car is running right - it maintains the 18-20 inch pounds required for booster. Booster holds 21lbs of vacuum overnight. So booster is good from that aspect. With the booster under vacuum I bled the front brakes old school - pump and hold. I did this to check pedal travel. Pedal drops perhaps 1 inch when I bled the caliper - same reaction both sides. I don't get a lot of fluid but was able to flush out the front system with new fluid. I went to the back as long as I had a willing partner to push the pedal - ugh back brake line clogged no back brakes - will swap that line out and let folks know results.
 
That's why I asked about bleeding the brakes post #7.

Did you pre-adjust the rear shoes to a slight drag?
Could be the slave cylinde(s)r are stuck or the rubber hose between the body and axle has collapsed.

Check the front calipers for symmetry, have someone "lightly"press the brake pedal while you observe the caliper for movement. The pads should come in contact with the rotor at the same time. Press full pedal,: are you getting equal movement, not favoring one pad or the other?
 
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Bewy said:
Any update?
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Sorry for the delay. I'm still working through the brake system and have ordered a new master but won't be able to install until May. I've traded my wrenches for Golf clubs and have headed south for the winter. To date: I replaced the line from rear axel to body and readjusted manual prop valve (keep rear brakes from locking up) and this improved braking performance - got 20% braking power now that rear brakes work. As stated before I put a new booster in and it felt like I was not getting assist - I'm not sure about that at this point - it just might be it feels different - harder pedal and less movement to engage. I'm still not getting the braking power from the front that I believe I should so have ordered a new MC to complete the brake upgrade - not at all sure I need one but ordered the matching MC to the booster from RIGHTSTUFF so they are designed to go together -
 
I have been working on mine and found both rear wheel cylinders are leaking. Don’t know if that is part of the issue, more to come…..
 
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