Brake Pedal Diagnostics

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340inabbody

340inabbody

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Hi getting my car running after long restoration and rear drum rebuild. Brake pedal is hard when pressed with engine off but goes to the floor when car is running.

Automatic transmission, front power disk rear drum. 10Bar vacuum…

What should I be looking at first?

Thanks!
 
Anyway.....the cam wouldn't make the pedal go to the floor, BUT, lets get specific. Does the pedal actually TOUCH the floor? OR are you bein AHHNOLD and givin it all you got? Because I can make "most" vehicles with power brakes pedals touch the floor if I really, really want to. The booster gives assist which makes the pedal easier to depress. So give us some specifics.
 
RustyRatRod said:
Anyway.....the cam wouldn't make the pedal go to the floor, BUT, lets get specific. Does the pedal actually TOUCH the floor? OR are you bein AHHNOLD and givin it all you got? Because I can make "most" vehicles with power brakes pedals touch the floor if I really, really want to. The booster gives assist which makes the pedal easier to depress. So give us some specifics.
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Light pressure. It just sinks to the floor. I don’t believe it’s the lower vacuum but I am not sure. Cam is unknown unfortunately. But it worked prior to my trsto and other than rear drum brake rebuild nothing else changed.
 
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340inabbody said:
Light pressure. It just sinks to the floor. I don’t believe it’s the lower vacuum but I am not sure. Cam is unknown unfortunately. But it worked prior to my trsto and other than rear drum brake rebuild nothing else changed.
Click to expand...
No, lower vacuum makes it tougher to operate power brakes. Tell me this. How far down can you mash the pedal with no vacuum assist?
 
From my first post “Brake pedal is hard when pressed with engine off but goes to the floor when car is running.”
 
340inabbody said:
From my first post “Brake pedal is hard when pressed with engine off but goes to the floor when car is running.”
Click to expand...
Yeah, I got that. But "HARD" could mean it just has more resistance. I'm trying to figure out if you may have a hydraulic issue here. How far down can you mash the pedal with the engine OFF? Can you force it to the floor or close? If so, this is likely a hydraulic issue.
 
If the order can go to the floor the fluid has to be going somewhere.

If it's not leaking, the master must be bypassing.

Pop the cover off and with the engine running have someone watch for fluid flowing into or out of the front or rear resovour
 
RustyRatRod said:
Yeah, I got that. But "HARD" could mean it just has more resistance. I'm trying to figure out if you may have a hydraulic issue here. How far down can you mash the pedal with the engine OFF? Can you force it to the floor or close? If so, this is likely a hydraulic issue.
Click to expand...
Feels fine firm and not to the floor.
 
I am going to bleed them to make sure there is no air in the system. I’ll bet that is the problem. Will let you guys know how it goes. I got a list of issues I am working through and its freaking hot here in Phoenix AZ.
 
If you have a hard pedal [ engine off ] & the pedal goes to the floor [ engine running ], you have air in the system.
 
there's so many possibilities here. the pedal generally drops a little once the car is on but excessive travel or sponginess is not acceptable.

if you haven't already, make sure that your rear brakes are adjusted properly. double check the bleed and make sure you have zero-point-zero air in the lines (this is probably the most common culprit, after leaks in the system). as dana mentioned, the master could be bypassing, this is common on a master with bad cup seals and seals can go bad from sitting. unless it's puking fluid out the back, then it's definitely the master. lastly, i'd suspect the booster.

thankfully most of those checks are easy, quick and free.
 
junkyardhero said:
there's so many possibilities here. the pedal generally drops a little once the car is on but excessive travel or sponginess is not acceptable.

if you haven't already, make sure that your rear brakes are adjusted properly. double check the bleed and make sure you have zero-point-zero air in the lines (this is probably the most common culprit, after leaks in the system). as dana mentioned, the master could be bypassing, this is common on a master with bad cup seals and seals can go bad from sitting. unless it's puking fluid out the back, then it's definitely the master. lastly, i'd suspect the booster.

thankfully most of those checks are easy, quick and free.
Click to expand...
Thanks yes I have had bad luck with so many new parts from sitting for 3+ years.

New ignition switch went bad
NSS went bad
New aluminum radiator leaked needed to be replaced
Rear brakes locked up and needed rebuild
System no charging need to check my VR today if I can.
Engine backfiring maybe valve adjustment but I don’t have cam specs…..it’s a solid lifter.

So yup lots of issues to deal with and its 115F in my garage…..no fun

But my bodywork and paint came out great lol

IMG_2383.jpeg
 
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Inertia said:
Adjust the rear brakes .
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Im curious about your suggestion. I adjusted them when I rebuilt them a week or so ago. It’s a mechanical adjustment based on pad to drum separation. Are you thinking that they are too loose causing excessive travel?
 
That car cannot be fixed. You need to ship it up here to me

I would adjust both/ all 4 drum brakes up tight, bleed the brakes, and see what that gets, then slack off the adjusters until they rub "lightly" and go from there.

If you are unfamiliar with drum brakes they can be quite deceiving, as simple as they are, AKA if the drums are worn or been "turned" and the shoes new, the shoes will effectively be too small for the now larger drums. Back in the day, you had the shoes "arced" which is a precision grinder that grinds off lining so that the shoes then match the drum diameter.
 
67Dart273 said:
That car cannot be fixed. You need to ship it up here to me

I would adjust both/ all 4 drum brakes up tight, bleed the brakes, and see what that gets, then slack off the adjusters until they rub "lightly" and go from there.

If you are unfamiliar with drum brakes they can be quite deceiving, as simple as they are, AKA if the drums are worn or been "turned" and the shoes new, the shoes will effectively be too small for the now larger drums. Back in the day, you had the shoes "arced" which is a precision grinder that grinds off lining so that the shoes then match the drum diameter.
Click to expand...
Yes I am not well versed on drum brakes. I think I did one or two sets in the 70’s. I set my shoes on the loose side doing the opposite of what you described. I figured I could tighten them as I go as I didn’t want them locked up. I finally got the car up on stands and will adjust the rear brakes and flush and bleed them in the morning …
Thanks for the great background and tips!
 
340inabbody said:
Im curious about your suggestion. I adjusted them when I rebuilt them a week or so ago. It’s a mechanical adjustment based on pad to drum separation. Are you thinking that they are too loose causing excessive travel?
Click to expand...

If you put in new pads and shoes, you won't get pedal till the 4 front pads are in full contact with the rotor, so that can take a bunch pumping, and if the rear shoes are out of adjustment, it can take a buncha pedal movement/pumps to get un-adjusted shoes out to contact the drums, so no pedal till then.

Lotta folks complain about low/no pedal, and rear brake adjustment is commonly the issue.

Bleeding - If you set the emergency on, that'll move the shoes out to the drum and keep them there, and the first coupla pumps will move the wheel cyl pushrods out to the shoes and stay there cuz the springs can't return them, then you will get pedal.
(If the wheel cylinder pushrod missed it's place in the shoe, that will cause no pedal.)

Good luck
 
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