Rear Brakes Locking up.

[skokie]

I noticed today that my rear brakes are locking up before my fronts. Is this normal or do I need to adjust my fronts to match the rears locking up point?

Thanks

[woodridge]

Need more info, by your post I take it has front drums.
Possibilities: Rear brakes out of adsustment
Master cylinder bad
Pressure differential valve moved to block fluid to rear brakes
Rear wheel cylinders frozen
Hope this helps

[skokie]

Yep I left a few things out
Ok here we go.
New power brake booster, new drums, shoes and wheel cylinders all around. New master cylinder. Master was bench bled before install.

[67ChevyRedneck]

dumb question, but did you get the correct master cylinder? If you have a drum/drum combo the reserviors should be equal in size. The one that has the bigger front reservior is for trucks with disks. Do you have a proportioning valve as well? Did you just add the power brakes?

[skokie]

Originally Posted by 67ChevyRedneck

dumb question, but did you get the correct master cylinder? If you have a drum/drum combo the reserviors should be equal in size. The one that has the bigger front reservior is for trucks with disks. Do you have a proportioning valve as well? Did you just add the power brakes?

The reservoirs are the same size. Checked the wheel cylinders and they are correct to.

[67ChevyRedneck]

Originally Posted by skokie

The reservoirs are the same size. Checked the wheel cylinders and they are correct to.

Well shoot, that was my only idea as to what could be wrong.

I guess my only other suggestion would be to check an see how tight the front brakes are in relation to the rear when you jack up the truck an pull the tires... (maybe you have the backs adjusted too tight and the fronts too loose??) Try rotating the drums to see how easy/hard they move. I'm just tossin ideas around, not tryin to say you don't know how to install brakes or nothin like that....

[chevymad]

Originally Posted by 67ChevyRedneck

Well shoot, that was my only idea as to what could be wrong.

I guess my only other suggestion would be to check an see how tight the front brakes are in relation to the rear when you jack up the truck an pull the tires... (maybe you have the backs adjusted too tight and the fronts too loose??) Try rotating the drums to see how easy/hard they move. I'm just tossin ideas around, not tryin to say you don't know how to install brakes or nothin like that....

Actually it's opposite of what you think. If the drum brakes are adjusted too loose it will cause them to lock up. Drum brakes are self actuating. As the shoes bite into the drum they turn and cam-lock in harder. If the shoe's are too sloppy it causes this cam action to be severe. This action is why you can get away without a booster easier when you have drum brakes.

I see you allready have new drums too. Drums that are worn out too large will also cause lockups. Does it lockup all the time or only first stop or 2 in the morning? The linings they make now absorb moisture when they sit overnight. This swells them up, makes them softer and grabbier. If this is the problem you'll have touchy brakes the first couple stops in the morning, then once the lining dries out they'll be fine.

[JRB]

When I got my 69 Short Step it was doing the same thing, I swapped lines on the master cylinder and it started working correctly. If you look at the wheel cylinders on these trucks you will find that the front wheel cylinders are different than the back ones. I hope yours is as easy to fix as mine was.
JRB

[JRB]

Sounds like you may have to get an adjustable Proportioning valve and pinch it off a bit to the rear. I dont know enough about the Master cylinder to know if more pressure goes out of one resevoir than the other. Good luck with your problem.
JRB

[67ChevyRedneck]

Originally Posted by JRB

Sounds like you may have to get an adjustable Proportioning valve and pinch it off a bit to the rear. I dont know enough about the Master cylinder to know if more pressure goes out of one resevoir than the other. Good luck with your problem.
JRB

I don't think drum brakes have a proporting valve, do they? Do power drums have them? My 67 with drums didn't have one

[JRB]

Unless I am mistaken you can get a aftermarket proportioning valve and put it in line to the rear brakes. In all the experience I have had with drum brakes I have dealt with single master cylinders up until I bought my 69 Short Step. I did have the same problem as you are having on mine and like I said when I changed the lines on the proportioning valve which mine has it stopped locking up the rear brakes. Hope this helps.
JRB

[skokie]

Brakes are like this all the time. Doesnt go away after the first few stops.

Thanks

[tcoop68]

First question,had your brakes on your truck been doing fine and then acted up, or did it act up after you did your brake overhaul?you said you put a new power brake booster on did your truck already have power brakes?because if it did'nt and you put one on you would have to get a matching proportioning valve to that year of power booster.Most power brake trucks had front disk does yours?I know we can get your problem figured out.

[skokie]

Truck needed brake overhaul and I just said what the heck lets do it all. As far as I knew this was the original stuff I was dealing with My truck did not have power brakes on it. I added that. My truck has drums all around. No disks.

[Longhorn Man]

There is no proportioning valve on a drum/drum truck, and it wouldn't matter if it was power assist or not.


How easily do the rears lock up? From your original post, I gather that it is under heavy braking, and the rears lock up just before the fronts.



I've never driven an older non ABS truck that did not act like that.

[skokie]

Originally Posted by Longhorn Man

There is no proportioning valve on a drum/drum truck, and it wouldn't matter if it was power assist or not.


How easily do the rears lock up? From your original post, I gather that it is under heavy braking, and the rears lock up just before the fronts.



I've never driven an older non ABS truck that did not act like that.

Andy
I would say that they lockup just before the fronts. I cant judge it in seconds its not like that. I would say more like as the pedal travels toward the floor.
Thanks
BTW hows that caddy motor holding up?

[Longhorn Man]

The caddy is holding up beautifuly. Best lump of iron I've had.