Those of you who converted to rear disk, can you feel much of a difference?

[WyattTX]

I see many talk about rear disk conversions here. All the factory front disk/rear drum trucks I've driven seem to have great stopping power (if the truck does have oversized tires.) The only four way disk trucks I've driven was a gmt800, and a 3rd gen Dodge 1 ton. Both of those stop good, but they are also ABS, so they may not be an accurate comparison.

Also, do the conversion kits use any parts interchangeable with the front disk? Thanks.

[MikeB]

Good question. I've done several front disc conversions but none at the rear. However, I know the main advantage discs have over drums is not stopping power per say, but standing up better to repeated heavy braking. But if your rear drums are in good working order, I doubt you'd notice any difference during everyday driving, unless you live in the mountains. Also having 12" or larger diameter rotors & calipers (which probably requires 16" or larger wheels) would probably increase stopping power over 11" drums. (Larger diameter results in more leverage when the pads clamp the rotor.)

But even if you get 20% more stopping power at the rear from a proper conversion, that's only 5%-10% more stopping power overall, because front brakes do most of the work.

For optimum drum brake stopping power, make sure 100% of the shoe surfaces are contacting the drums. With freshly turned drums and new shoes that's not going to happen because the arcs are different! And good luck finding a shop with a machine that can "arc in" new shoes. They are long gone with 10 cent Cokes. So that means you need new drums to go with new shoes for 100% contact. Also, make sure the rear brake hose is in good shape. If it shows any outward signs of aging, it may be expanding under pressure.

[MikeB]

I just found this info in a Haynes manual:

Drum brake advantages

• Brake shoes today are still being used. Here are some advantages that drum brakes have over disc brakes:
  
• Drum brakes can provide more braking force than an equal diameter disc brake.
• Drum brakes last longer because drum brakes have increased friction contact area than a disc.
• Drum brakes are cheaper to manufacture than disc brakes.
• Rear drum brakes generate lower heat.
• Drum brakes have a built-in self energizing effect that requires less input force (such as hydraulic pressure
• Wheel cylinders are simpler to recondition than with disc brake calipers.
• Brake shoes can be remanufactured for future use.
• Drums have slightly lower frequency of maintenance due to better corrosion resistance.

Drum brake disadvantages

• Excessive heating can happen due to heavy braking, which then can cause the drum to distort, and thus cause vibration under braking.
• Under hard braking, the diameter of the drum increases slightly due to thermal expansion, the driver must press the brake pedal farther.
• Brake shoes can overheat to the point where they become glazed.
• Excessive brake drum heating can cause the brake fluid to vaporise.
• Grab is the opposite of fade: when the pad friction goes up, the self-assisting nature of the brakes causes application force to go up. If the pad friction is enough, the brake will stay engaged due to self-application, even when the external application force is released.
• Another disadvantage of drum brakes is their relative complexity.
• Maintenance of drum brakes is more time-consuming, compared to disc brakes.

Also, you didn't say why kind of truck you have, but if it's a C10, here's a quality rear disc kit:

https://www.mpbrakes.com/pc_product_...2A8B6A33EF56CB

[cadillac_al]

If you like a hokey half ass parking brake then rear discs are for you. Rear discs work nice enough but the parking brake is always a headache. At least a drum brake emergency brake can be used in emergencies. Disc brake park brakes barely hold on level ground in neutral.

[Ziegelsteinfaust]

If you have decent sized with good components drum brakes you will not likely feel any difference. They will how ever last longer with out fade under heavy breaking.

If you have a single stage master cylinder the breaking will feel better due to proper front rear bias.

My friend had a 63 C10 with very small, but practically new drum breaks. The stopping distance was not much better with 11.75 C10 hd front breaks, and El Dorado rear disks. Something like 5' at best between the few times we took measurements. All within the margin of error based on his reaction times. Although on the 3rd time he could really feel the breaks starting to get soft. So heat soak was starting to appear in the breaks, but not to where it was effecting stopping power on that test run.

There are two other things people never due for there breaks that can dramatically increase effectiveness. Stickier tires, and light weight wheels. The tires help with contact patch, and the light weight rims help with the breaks overcoming the wheels inertia. Also run reasonably wide tires. I see to many people running stupidly thin tires for what ever reason

[WyattTX]

Thanks for the insight. I wasn't really looking to convert anything, I was just wondering why everyone else does it. Also, if you go all drums, or all discs, is a proportion valve still needed?

[MikeB]

Originally Posted by WyattTX

Also, if you go all drums, or all discs, is a proportion valve still needed?

GM didn't use a prop valve in my 69 C10 truck with drum/drum. Can't say if that's true for all vehicles.

As for disc/disc, I think the only reason to use a prop valve would be if the rears have a tendency to lock up, as might be the case when using aftermarket parts that aren't balanced for a specific vehicle's front/rear weight bias. In that case, I would use a manual adjustable valve, instead of a generic valve designed for a specific vehicle.

As an example, I had a disc/drum 55 Chevy with a 7.5" 10-bolt axle originally used under lighter weight G-body cars. With its wimpy 9" drums, my best brake performance was with the manual valve (installed by the previous owner) cranked to max pressure to the rear. So it served no purpose at all. Now with 11" rear drums and shoes, it might have been needed to prevent premature rear lock-up.

Start here about 2/3 down the page under Valves FAQ:
https://info.mpbrakes.com/faqs

[MikeB]

Originally Posted by cadillac_al

If you like a hokey half ass parking brake then rear discs are for you.

Have a friend who bought a 56 Chevy sedan that had a 9-inch rear end from a late 70s Lincoln Versailles. The brakes work fine, but parking brakes are almost worthless and just about impossible to adjust. Can't tell you how many hours we have spent trying to get them to work. Same may be true for other parking brake systems where pads are clamped to the rotor by turning/ratcheting the piston. Some Hondas, on the other hand, use that system and work fine.

He may eventually go with a kit that uses rotors with integral drums and small parking brake shoes. But he's also considering a Speedway drum brake kit! I hear those Ford drum brakes originally used in trucks and 4,000 lb full-size cars work very well.

[Tom]

Originally Posted by WyattTX

Both of those stop good, but they are also ABS, so they may not be an accurate comparison.

Unless your comparing panic stops where the brakes are locking up then yes, it is a direct and accurate comparison.
And no, changing the the rear brakes won't make you feel a difference in power unless your current brakes can't lock up the tires [which any factory system should do]. I put disk rears on my 98. It stops the same.

[SunSoaked]

A hydroboost swap is much more effective and likely less costly.

[WyattTX]

Originally Posted by SunSoaked

A hydroboost swap is much more effective and likely less costly.

I thought about that, but those are expensive. Would you be able to reuse the factory proportion valve?

[kipps]

Originally Posted by WyattTX

I thought about that, but those are expensive. Would you be able to reuse the factory proportion valve?

I pulled a complete hydroboost assembly, all hoses, mounting plate, pedal, pedal bracket, and pushrod from a local u-pick yard for $125. Came off a '82 diesel, and is bolting onto a '87 gas. Paid $50 for a rebuild kit and installed it myself. Don't have the truck running yet, so can't guarantee all is fine, but it should be.

I'll be using the factory brake system other than the booster. I did not pull the pump from the donor, because it had the remote reservoir type, and I doubted that would fit on a LS engine. I don't intend to bother tapping both returns into the pump reservoir. Instead I'll just tee the return lines together. May also install a PS cooler that I got with my LS donor vehicle. If so, the return lines from the hydroboost and the steering box will be teed together before they go to the cooler.

Further, being an 80's truck, my truck uses metric o-ring fittings on the lines. The booster I bought is the same way. The LS power steering pump uses the same fittings. I can buy new high pressure lines for a 80's hydroboost truck, and they should fit perfectly between this mishmash of '82, '87, and 2010. The low pressure lines will need to be fabbed, but that's easy.

Earlier hydroboost units had flare fittings instead of o-rings. Research the year breaks if it matters to you.

Am told that all hydroboost units are the same, other than the fitting type and a few other minor things. Just need to do some more fabrication for the mounting bracket and pushrod, if pulling from a non-squarebody source. Theoretically most Astro vans have hydroboost, and it's supposed to be easy to get it off of them.

Best rebuild instructions that I've found are here.

[WyattTX]

Originally Posted by kipps

I pulled a complete hydroboost assembly, all hoses, mounting plate, pedal, pedal bracket, and pushrod from a local u-pick yard for $125. Came off a '82 diesel, and is bolting onto a '87 gas. Paid $50 for a rebuild kit and installed it myself. Don't have the truck running yet, so can't guarantee all is fine, but it should be.

I'll be using the factory brake system other than the booster. I did not pull the pump from the donor, because it had the remote reservoir type, and I doubted that would fit on a LS engine. I don't intend to bother tapping both returns into the pump reservoir. Instead I'll just tee the return lines together. May also install a PS cooler that I got with my LS donor vehicle. If so, the return lines from the hydroboost and the steering box will be teed together before they go to the cooler.

Further, being an 80's truck, my truck uses metric o-ring fittings on the lines. The booster I bought is the same way. The LS power steering pump uses the same fittings. I can buy new high pressure lines for a 80's hydroboost truck, and they should fit perfectly between this mishmash of '82, '87, and 2010. The low pressure lines will need to be fabbed, but that's easy.

Earlier hydroboost units had flare fittings instead of o-rings. Research the year breaks if it matters to you.

Am told that all hydroboost units are the same, other than the fitting type and a few other minor things. Just need to do some more fabrication for the mounting bracket and pushrod, if pulling from a non-squarebody source. Theoretically most Astro vans have hydroboost, and it's supposed to be easy to get it off of them.

Best rebuild instructions that I've found are here.

That seems straight forward enough. The only problem I see here, is that you'd have to keep track of all the parts you bought, when it came time for replacement.

[SCOTI]

Originally Posted by kipps

I pulled a complete hydroboost assembly, all hoses, mounting plate, pedal, pedal bracket, and pushrod from a local u-pick yard for $125. Came off a '82 diesel, and is bolting onto a '87 gas. Paid $50 for a rebuild kit and installed it myself. Don't have the truck running yet, so can't guarantee all is fine, but it should be.

I'll be using the factory brake system other than the booster. I did not pull the pump from the donor, because it had the remote reservoir type, and I doubted that would fit on a LS engine. I don't intend to bother tapping both returns into the pump reservoir. Instead I'll just tee the return lines together. May also install a PS cooler that I got with my LS donor vehicle. If so, the return lines from the hydroboost and the steering box will be teed together before they go to the cooler.

Further, being an 80's truck, my truck uses metric o-ring fittings on the lines. The booster I bought is the same way. The LS power steering pump uses the same fittings. I can buy new high pressure lines for a 80's hydroboost truck, and they should fit perfectly between this mishmash of '82, '87, and 2010. The low pressure lines will need to be fabbed, but that's easy.

Earlier hydroboost units had flare fittings instead of o-rings. Research the year breaks if it matters to you.

Am told that all hydroboost units are the same, other than the fitting type and a few other minor things. Just need to do some more fabrication for the mounting bracket and pushrod, if pulling from a non-squarebody source. Theoretically most Astro vans have hydroboost, and it's supposed to be easy to get it off of them.

Best rebuild instructions that I've found are here.

HydraBoost pedals are different. Did you swap pedals too? The brake pedal (leverage) ratio is different between the two. Obviously, the brakes will still function but not as intended.

[1976gmc20]

The fellow we buy hay from who also does some mechanic work says that rocks like to get stuck between the rear calipers and the wheel. If you have your windows rolled up and AC and radio on then you don't hear it until the wheel is ruined.

[Tom]

Again, hydroboost isn't going to make the truck stop better unless you can't lock up the wheels currently. It will change the brake pedal feel, but power is dictated by the calipers, rotors and pad material more then anything else.

[kipps]

Originally Posted by Tom

Again, hydroboost isn't going to make the truck stop better unless you can't lock up the wheels currently. It will change the brake pedal feel, but power is dictated by the calipers, rotors and pad material more then anything else.

Agree. Hydroboost is simply swapping the booster side of the master cylinder. When I was driving my truck(still in the middle of an engine, etc, swap right now), I could not lock up the wheels. The tires were oversize, which may have contributed, but it often didn't feel safe. Not to say I wasn't having other problems, but I figured it's easier to swap to a HB now rather than later. I have JB7 brakes, so I'm hoping the addition of HB will make it a breeze to stop this thing.

[Honest Bob]

I have a k20 with the vacuum brakes and a c30 with the hydroboost brakes.

Yeah the k20 brakes do work ok but the c30 can and will lock em up on command. I cant get the c30 to move with the parking brake on, feels like its chained to a house. If you are running bigger tires I'd go hydroboost for sure.

[cadillac_al]

Well I hate to always be the Negative Nellie but I never liked hydroboost when I had it on an Olds 98. A panic stop would stall the engine every time. Somehow the power steering pump gets so hard to turn during a panic stop that it stalls the engine. Maybe a higher idle speed would cure it but I don't like a high idle either. The more we discuss it the better the stock drums sound.

[WyattTX]

Honestly, after much research, and reading posts in this thread, I think a properly sized disc/drum set up is more than enough to stop a vehicle. 4 way disc is just cleaner looking, and easier to service.

[kipps]

Originally Posted by WyattTX

... 4 way disc is just cleaner looking, and easier to service.

I agree with you there. However personally, I'd only really consider rear discs if I can go with a driveshaft brake. To me, elegance means a floor-mounted handbrake with direct mechanical linkage to a driveshaft brake. Problem is, I don't think those were ever mounted behind a transfer case. Thus, would have to fab up a system from scratch, or go with a divorced TC and a brake on the transmission.

[hatzie]

I have/had Hydroboost on more than a few pickup trucks. Seven of them IIRC. The brakes all behaved better than vacuum boosted brakes.
None of them stalled when applying the brakes.

Something was very wrong with that Oldsmobile above and beyond the Hydroboost brakes.

The stock rear drum front disc brakes work quite well on the squarebody trucks.

[WyattTX]

Can boosters and master cylinders can be swapped in GM vehicles of the same vintage? I've heard of some people trying to adapt the Corvette C3 setup on the other vehicles.

[Tom]

If you just want a harder pedal then swap the master for a different bore diameter.
On my 98 I ended up with an 02 tahoe master cylinder to get the pedal feeling right. That was after steel brake lines all around, high GVW 3/4ton calipers [with the bigger piston] and 3/4ton pads [oh so slightly larger], a new oem master and brake booster, and finally a NBS rear disk setup using tahoe 2 piston calipers vs the 1piston truck calipers. Truck always stopped fine, I just hated the pedal feel. New, different vaccum master fixed her right up.