Brake question (long)

[joescott]

Hello all
I have a 1967 GMC ½ ton lwb pu. In November I got the truck back on the road, but the brakes did not seem to work properly. They would eventually stop the truck but were very hard. The truck has 4 wheel drums by the way. I have replaced the MC, all 4 wheel cylinders, all shoes, all rubber lines, and front hard lines. Because the truck has all drums, it uses a dist block instead of prop valve, but I cleaned it out anyway. I have bled the system using the pump method, also used a vacuum pump at the wheel cylinder and pressure bled from the wheel cylinders. I adjusted the brake adjusters to the point of a hard to turn wheel. The pedal is very hard with no signs of sponginess. All four wheels will stop when the truck is on jack stands. When on the road the truck still feels hard to stop. It does stop but I worry the seat mount bolts will break with the effort I have to push on the pedal.
Today I am going to a junkyard to find a booster to convert to power brakes. I have read on this board that some trucks use the front reservoir in the MC to run the rear brakes. When pressure bleeding I confirmed the rear brakes are connected to the rear reservoir and the front are connected to the front. Is this correct?
Thanks in advance.

Joe

[bandit1]

Maybe you have a master for power brakes, i think they have a different size bore.

[WorkinLonghorn]

You didn't say if you turned the drums.How was the surface of these? Does the truck pull to one side at all? Have you re-checked those new shoes to see if they are getting oil on them from a leaky seal?
>>You need to make sure they are dry and very clean! If so I would say you are on the right course of action upgrading to vac-assist. Verify that you have the correct parts,like bandit1 suggested. gl

[PanelDeland]

I would check the drums/shoes too.You shouldn't have that much trouble stopping your truck.If the master is wrong you may have a problem there but if you have rebuilt the brake system it should stop just about like disc without power.It will get worse as they fade from heat during repeat stops but they should work well cool.I doubt that the vacuum booster will do much other than lower the pedal pressure.I would also check the master cyl to make sure it's the correct drum/drum for the application.

[Americanrider]

Joe, your right. On the half tons it's front res to front brakes/ rear res to rear brakes. On the 3/4 & 1 tons it's front res feeds rear brakes & rear res feeds front brakes.

[mrein3]

Originally Posted by joescott

Hello all
stuff deleted...
The pedal is very hard with no signs of sponginess. All four wheels will stop when the truck is on jack stands. When on the road the truck still feels hard to stop. It does stop but I worry the seat mount bolts will break with the effort I have to push on the pedal.
...stuff deleted
Joe

It sounds like you have done all the right stuff for your brakes. New shoes, got the air out of the system, adjusted the shoes so there is a touch of resistance in the wheel when you try to turn it.

Now it is time to face the facts. Drum brakes suck. There is a reason why everything now comes with at least front disks. They work better.

On my 72 Chevelle, I drove it around for 15 or so years with 4 wheel, manual drums. I could NOT lock up the wheels on the highway. Then AFTER that hard stop, there was just nothing left because everything was hot. Then came the time to junk my 1971 Monte Carlo. Montes of that era came with power front dick brakes while on Chevelles it was an option. That is why you never see Montes with front brakes in the yard. The spindles are a direct bolt in for other A-bodies. AND THEY WORK BETTER.

This is where all the "drum brake club" folks say "yeah well my drums work". But sadly some folks just don't understand physics.

To make a really long post short, go get a disk brake conversion of some kind. There are millions of conversion kits available. There are millions of posts in this forum you can search and read up on that detail swapping the front out of a 73-87 in. The difference in my Chevelle was night and day.

You're never going to get the same stopping power out of a set of drums that you will out of a set of front disks.

[WorkinLonghorn]

I realize this is off-topic but regarding:

This is where all the "drum brake club" folks say "yeah well my drums work". But sadly some folks just don't understand physics.

I agree that disks are great and that some drum brakes are inadequte,but the drums on my 3/4 will stop my truck from 60 MPH with 4000 lbs in the bed on a very short,down-hill freeway off-ramp with about 20 lbs pressure on the peddle. I keep them in perfect comdition.But I haven't tried taking that 4000 lbs. down any long mountain passes.Maybe then I would want disks but I really don't see any problem with properly designed and maintained drums,at least on the 3/4 with vac.assist. What "physics" am I missing here?

[joescott]

Thanks to all for the replies.
I did have the drums turned and they looked very polished. The people that turned them did not mention if they were beyond tolerance or not. I wondered if the curve of the drum did not match the curve of the new shoes. I was told by a parts counter guy they used to have a grinder to mate the surfaces. He had not seen one in years however. I was at that store to ask about the possability of a wrong master cylinder such as one for power brakes. This is an old store and he referenced his "books" instead of a computer, and indicated I had the correct MC.
I went to the junkyard and pulled a booster of a 74 chevroloet PU. I got a vacume line from a van to make sure I had plenty. After drilling additional holes and removing parts of the bracket for clearance, I successfully mounted the booster. I lengthened the rod and hooked up the pedal.
With engine off checked to make sure brakes were still available. The pedal actually felt better. Started the motor and touched the pedal, itfelt like it was going right to the floor!! It started to slow about half way and stopped about two thirds down.
I am a doom and gloom kind of guy. This helps me accept failure because I expect it to happen. The pedal did not feel like it would stop the truck and I was not going to let this get me down. Time to back it out of the garage. press the brakes and the truck came to a quick and effortless STOP!

WOOOO HOOOOO!!!!!!

I GOT BRAKES. sure a panic stop will try and throw me into the ditch but the truck stops and I am so happy.

Next paycheck I will see to it my name turns blue on this wonderfull forum.
Thanks to all.

Joe

[ToddSS]

Ah.... the old drum vs. disc debate.

As a brake and controls engineer up here in the motor city, dispelling myths is a favorite pastime of mine. That, and I feel like a good long reply to this to make the day go by quicker.

Unfortunately, the laws of physics mainly dictate motion as a function of distance, acceleration, velocity, mass etc. Brakes deal in energy transfer though friction which is turned into heat transfer, so physics in the brake world is not referenced a whole lot.

Yes, disc brakes are used exclusively on the front wheels of todays vehicles and on most of the rears but do you know why? It is not because they are a completely superior brake design. If they were, then they would have dropped drum brakes from all vehicles back in 1970 or so when they realized that most vehicles should have them on the fronts. But they still use them today. If they were simply better, they would have dropped them all-together long ago like the mechanical proportioning valve which died out in all ABS-equipped vehicles when they realized that pulsing the isolation valves in the ABS circuits of the rear of the vehicle did the same thing with software and no hard parts were needed anymore.

Drum brakes are actually more efficient of a design with regards to effective stopping power. Force is a function of diameter (or moment arm) just like a torque wrench and drum brakes actuate the braking force at the outtermost point of the diameter, while disc brakes don't even come close to the farthest point of the center of rotation, spreading the forces all along the face diameter of the rotor. Also, and this should be obvious when you think of it, the drums have a more balanced force because the shoes contact the drums opposite of each other, creating a balanced force with respect to the axis of the spindle and not one moment like a disc caliper.

Why are they not used all over the place then? Becasue of the inability to cool effectively. They are great for a couple of stops but because they are in an enclosed space, they have a tendency to fade very quickly compared to disc brakes and can't dissipate the heat. In today's stop-and-go environment and with drivers getting more aggressive every day, brake fade is no longer acceptable as it once was. Aluminum finned drums have been used with some positive results but they still cannot remove heat quick enough from the system. It all gets back to heat transfer. Drums cannot remove the heat from the system during constant or frequent abuse situations and the heat builds up and bad things happen.

Why are they still used at all? One reason. Fuel efficiency. Some will say they are lower cost and sometimes they are depending on comparable disc designs but mainly because the shoes retract from the metal surface and there is no parasitic drag on the drum, they are far superior to discs which have to maintain contact with the rotors continually. It's surprising how much drag disc brakes really create.

Now, for a couple of data points, I have modified many brake systems on project vehicles over the years and have also seen many, many brake systems turned into dangerous affairs with people changing components in an unsafe way by improperly sizing the system with respect to master cylinder piston volumes compared to caliper piston volumes (like putting dual pistons on the fronts from single without giving consideration to master cylinder diameters) and also messing around with the rear brakes or prop valves resulting in a rear-biased vehicle.

I recently did a 1952 Ford F1 with a flathead V8 project. Don't worry, I got the Ford stuff out of my system then. The truck had 4 wheel drums that were 8 lug. I wanted to go 5 for wheels and stuff, so I had to convert anyway, so I went with 5 lug discs all around. I was planning on going with power brakes, but was unable to find a booster that would work with the undercab mounting. I changed to the correct master cylinder for the chosen brakes. When I got it all together and bled completely, the brakes worked great, but there was still excessive pedal force needed to stop the vehicle. This was simply a function of the manual brakes, even though they were 4 wheel discs. The advantage of the vaccuum assist can't be understated when modifying/improving old vehicles.

By comparison, just this last weekend, I completed my manual to power drum brake conversion on my '68 C10. I was debating whether or not I would need to then go with the front disc conversion to get the results I wanted but I knew I was going to do the booster, so I did that first. Before doing any of this, I replaced all of the fluid and bled the whole thing because I have seen too many people do brake upgrades when their fluid was more than 10 years old and all it really needed was a brake bleed. The shoes were adjusted so the brakes were as good as they could get in stock form. I was not happy with them and the effort in general was entirely too high to stop the truck and way too high to keep it stopped at a light. It wasn't like I was thinking I was going to break the seat brackets like the first post in this thread but it was still too high.

After the booster, the first time I went to hit the brakes, I locked up all 4 tires going about 40 or so. It was a much more drastic improvement than I had hoped for and I was pretty happy with it! AT highway speeds, stopping effort is so much lower and so far, no indication of excessive fade. I could never have locked up my fronts before the booster. So most times when people have troubles with the 4 wheel manual drum systems, the real problem is the manual part, not the drum part.

Now, if you have a vehicle that has a good bleed, has all good working calipers or brake cylinders and has a booster and still does not have the ability to lock up at least the front brakes, then there is something else wrong with the system or it may in fact have undersized brake drums for the vehicle and an upgrade is the best course of action.

Otherwise, if you can lock up the brakes with moderate effort, the only real reason to upgrade to discs is if you are in traffic often or have other concerns that are brake fade related.

Just trying to maybe save you dudes some trouble and money. Don't get me wrong, disc brakes are definitely the way to go, but drums have gotten a bad reputation over the years and only partially deserve it.

Later!
Todd

[Sonny]

Big rigs still use drums (more contact area for one reason). If you seen the big trucks race overeas they use water cooled drum brakes.

[GMCBubba]

thanks for the input todd! justifies me sticking with drums for now. maybe booster in the future when i drive it more

[ToddSS]

Yeah, figure out a way to cool them and you're good to go!

Drag racers like 'em still these days because of the lack of frictional drag when you're not using them. Of course, the wife's not going to be too happy hearing that she has to get under the vehicle and adjust her brakes every year or so to make sure the drums are nice and snug and well-adjusted.

It's all plusses and minuses.