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No brake pressure in "drive"
https://slantsix.org/forum/viewtopic.php?t=31477		 Page 1 of 3
Author:  Karasik87 [ Fri Oct 03, 2008 6:03 pm ]
Post subject:  No brake pressure in "drive"
I had to replace the wheel cylinders on my rear brakes. The drum says 9". I got everything put back together and have bled the break lines. When i have the car started in park, I can feel pressure in the brake pedal. When I shift to drive, the pedal has no resistance. I am lost.
Author:  Joshie225 [ Fri Oct 03, 2008 7:50 pm ]
Post subject: 
Do you have power brakes? If so the booster check valve could be bad and the engine vacuum a little low. The pedal will sink a little in drive regardless, but only a little.
Author:  THOR [ Fri Oct 03, 2008 10:14 pm ]
Post subject:  Intarwesting...
How far down does the pedal go in park? Regardless, you should still have brake pressure in the front, and the pedal shouldn't sink all the way to the floor. If you have a booster, I agree with Josh, if not, then I would say that you have some adjustments to make.

What method did you use to bleed the brakes?

~THOR~
Author:  dakight [ Sat Oct 04, 2008 4:00 am ]
Post subject: 
Adjust the rear brakes according to the procedure in the factory service manual. Also be sure you put all the parts back in and in the right place. It's normal for the pedal to drop slightly when the transmission engages but if it's a drastic drop then something is wrong.
Author:  NewSlanter [ Sat Oct 04, 2008 10:01 am ]
Post subject: 
Dakight,

I'm not sure I understand why the brake pedal would drop when the transmission is engaged. Seems to me they're separate systems. My pedal ('70 Dart, 10" rear drums, disc front) feels the same regardless of speed selector position. I'm not saying that you're wrong, but I don't understand why it would happen. Could you explain?

Thanks,

Nat
Author:  THOR [ Sat Oct 04, 2008 12:04 pm ]
Post subject: 
Drum brakes are a much different animal than disc. When you swap directions on disc brakes, they dont care because they can't move. However, in a drum brake system, the shoe assembly inside the drum can rotate slightly inside the drum, because they are designed to be self energizing (meaning, once you have applied the pressure you need, they will apply a bit more to keep you from having to really stand on it to come to a stop... even though that part wasn't ever true, you still have to stand on it sometimes :roll: )

What this equates to is a movement of fluid pressures around the system, since the passenger and driver side drum brakes rotate the exact opposite direction given that the car is moving either forward or backward. Hence the drop of the pedal when you go into any forward gear. The reason they don't do the same when you go into reverse is because these drums were built with a self-adjusting mechanism. The idea was that you would drive in reverse at about 10mph, and then start applying the brakes... hard. Not lock them up, but apply them hard, and that would keep the shoes the appropriate distance from the drum. You can try that and see if it works, although I've only ever had it work for one car I've owned, and it was my '71 Dart with 9 inchers all the way around.

Joshie is correct saying that they should drop slightly, but no more than say about 1/2". Also, your pedal should start to engage the brakes no more than 1/2" after you step on the brake pedal. You may want to check and make sure you've still got enough brake fluid in there. Even after bleeding the brakes, once you use them, you've got to top them up again.

~THOR~
Author:  dakight [ Sat Oct 04, 2008 2:31 pm ]
Post subject: 
Yep, that just about sums it up.
Author:  Karasik87 [ Sat Oct 04, 2008 3:06 pm ]
Post subject: 
The guru i work with said to try rebleeding them. I did. Still nothing. WHen test driving down to the end of the street, I tried pumping the brakes. Sure enough, I got pressure. I don't have a FSM yet. I am getting one. But when the wheel cylinder completely failed, I couldn't exactly wait around. So, I am thinking I don't have power brakes. But I don't know. I'm still learning.
Also, the pedal never just sank to the floor. I had to push it down. There was no resistance.
Author:  Karasik87 [ Sat Oct 04, 2008 3:20 pm ]
Post subject: 
After checking the picture book... I don't have a brake booster. So that means I'm supposed to pump the brakes right?

* I hereby award myself the n00b cap.
Author:  VDART [ Sat Oct 04, 2008 6:57 pm ]
Post subject:  another check
There are lots of things -- to check-- many have been mentioned-- you also might have blown the seals out of the master cylinder or the front wheels cylinders-- when the new repairs were made-- you created a new weak link by fixing the old weak link. No pedal-- is the fluid level remaining constant after the bleeding of the brakes? Lawrence
Author:  THOR [ Sun Oct 05, 2008 2:14 am ]
Post subject:  Re: another check
Quote:
There are lots of things -- to check-- many have been mentioned-- you also might have blown the seals out of the master cylinder or the front wheels cylinders-- when the new repairs were made-- you created a new weak link by fixing the old weak link. No pedal-- is the fluid level remaining constant after the bleeding of the brakes? Lawrence
I agree. The old master cyliner should probably be replaced by now anyways, but if you are anything like me, I try to get the last minute out of every part. I forget how much new master cylinders are, but compared to the insurance costs of being in an accident, I think it's pretty cheap.

Here is one from Auto-Scone that is ok on price. Are your disc brakes factory? The disc brake master cylinder for a 1974 is much cheaper and you can find that one here.
I know that there any many who do not like reman parts, but I've had decent luck with the autozone stuff... not so much with CSK anymore.

~THOR~
Author:  dakight [ Sun Oct 05, 2008 4:53 am ]
Post subject: 
Get a new master cylinder, not a reman. I repeat, do not buy a reman; they are far too risky when you are betting your life, and someone else's life on them. New MC's can be purchased from a number of sources for less than $100.00... cheap insurance. If the cylinder bore is not scored or pitted you can easily rebuild it yourself at a very nominal cost.
Author:  Karasik87 [ Sun Oct 05, 2008 9:34 am ]
Post subject: 
so the first thing I did after rebleeding on saturday was go on a road trip. :) 3 full pumps gets me brake pressure and allows me to stop with relative ease. I don't know if thats normal for manual drums or not. Every car I have ever drove has had powered. I also know that my shoes are old and thin. I didn't replace them when replacing the wheel cylinders, because since I'm pretty new at fixing my own stuff, I try to limit how much is done at once, for ease of troubleshooting. And yes, I am planning to get a new master cylinder. Actually, I am planning to put new wheel cylinders in the front brakes too. Just cause. And I don't think it will hurt.
Author:  SlantSixDan [ Sun Oct 05, 2008 4:20 pm ]
Post subject: 
It is never, ever normal to have to pump the brakes. If you have to pump the brakes to get the car to stop, you need to stop driving the car immediately, until the problem is found and fixed. The car is very unsafe in its present condition; at any random time you could find that pumping the pedal no longer makes the brakes work. You almost certainly need a new master cylinder. When you bleed brakes with the pedal-pump method, the master cylinder piston travels beyond its normal range of motion. If the master cylinder is old, there's a bunch of roughness and grit in the area beyond the normal range of motion, so pushing the piston and its seals into this area shreds/grinds them, and then you get no brake application pressure unless you pump the brakes (for awhile, then you have none at all). Follow the good advice you got above, get a new (not "remanufactured") master cylinder.
Author:  THOR [ Sun Oct 05, 2008 8:27 pm ]
Post subject: 
Quote:
Get a new master cylinder, not a reman. I repeat, do not buy a reman; they are far too risky when you are betting your life, and someone else's life on them. New MC's can be purchased from a number of sources for less than $100.00... cheap insurance. If the cylinder bore is not scored or pitted you can easily rebuild it yourself at a very nominal cost.
I agree. I simply speak from the economy version of a fix. Since I have poorcollegestudent-itis, I sometimes can't afford a new one. If you can, do what dakight, and SlantSixDan recommend and get a new one.

~THOR~
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