80mm brakes
- Thread starter gerryg
- Start date
My coach had terrible brakes when I got it. When rebuilding them all, I found that they didn't have many miles at all on a previous rebuild. Just too long sitting, causing a lot of rusty bores and moisture laden brake fluid. I replaced shoes, wheel cylinders, larger wheel cylinders on middle axle, brake lines, Performance brake pads, combination valve, and master cylinder. I installed the 80MM calipers and spent an hour or two hand shaping the front brake lines to clear the CV boot and knuckle. It stops okay now, but two things may have changed my mind if I had known. 1st, the factory steel wheel will not go on the front with 80mm calipers. Darn. I have 6 American Eagle aluminum wheels and a steel spare. So if I have a flat on the front, I'll have to rotate a wheel off the back to the front and put the spare on the back. Also, I have exactly 1/4" clearance between the valve stem on the wheel and the caliper. Works so far. 2nd item, is that the larger volume of the 80mm pistons allow the brake pedal to travel farther down before encountering hard pedal. Driving 1000 miles with practically no brakes gives you great sensitivity to more pedal travel. I'm sure the 1-1/16" wheel cylinders on the front bogie axle add to this. I'd like to have someone with REALLY GOOD brakes drive mine and give me an opinion about how good mine are compared to theirs.
Thanks Greg, my coach is in a similar situation. It had been in a yard for 8 years, had been started and moved regularly (it was always in the way!) but never been out on the road. The brakes were low on fluid and there was some dry rusty looking stuff in the master. I'm going to redo them completely but just not sure which way to go.
regards
Gerry
regards
Gerry
Well, as I said, I may have not installed the 80mm had I known the steel wheel wouldn't fit. However, if I had 7 aluminum wheels instead of 6, it wouldn't be an issue. I installed a stock type master cylinder. Many folks use different MC's that have more volume, so the extra pedal travel may not come into play then.
One thing to note is that the master cylinder exterior is flooded with water every time it rains on the coach. It's conceivable that this contributes to moisture in the fluid reservoirs. I've been covering mine with a plastic bag. Some folks have put clear vinyl or aluminum shields attached to the body over the MC to drain the water away. You'll get varying answers on the calipers. Many are quite satisfied with a completely stock but well tuned factory brake system. Others say there is no comparison to 6 wheel discs. I say make it safe, then later you can upgrade if you so choose. If my coach had stopped well when I bought it, I would have a better base of comparison for you, but it was terrible, so anything I did was an improvement.
One thing to note is that the master cylinder exterior is flooded with water every time it rains on the coach. It's conceivable that this contributes to moisture in the fluid reservoirs. I've been covering mine with a plastic bag. Some folks have put clear vinyl or aluminum shields attached to the body over the MC to drain the water away. You'll get varying answers on the calipers. Many are quite satisfied with a completely stock but well tuned factory brake system. Others say there is no comparison to 6 wheel discs. I say make it safe, then later you can upgrade if you so choose. If my coach had stopped well when I bought it, I would have a better base of comparison for you, but it was terrible, so anything I did was an improvement.
My understanding is there is a boss on the 80mm calipers that must be ground down to fit inside the wheels. I don't have them, so I'm not sure if that is only for steel wheels or for both.
Maybe search over at GMCNet or GMCmhphotos.com
Maybe search over at GMCNet or GMCmhphotos.com
Tom Pryor and JimK have a lot of information. It is theirs and not original because the original numbers are all with bias tires.
Matt
Nothing really wrong with stock... but if you are replacing stuff, going to 80mm is so simple and cheap, and I believe there is some improvement.
wheel fitment can be an issue for some, but most rims fit fine. 16.5” stock and alcoas for sure are no issue with clearance, as is my dodge rims.
you have to grind the casting nub like the instructions say. But also a little bit of the knuckle as well.
I have a photo somewhere.
i am impressed with my 80mm. we installed them at a work rally. And there was nothing wrong with my stock brakes at that time. I am running performance friction pads and we properly bed them in. My coach has better braking.
wheel fitment can be an issue for some, but most rims fit fine. 16.5” stock and alcoas for sure are no issue with clearance, as is my dodge rims.
you have to grind the casting nub like the instructions say. But also a little bit of the knuckle as well.
I have a photo somewhere.
i am impressed with my 80mm. we installed them at a work rally. And there was nothing wrong with my stock brakes at that time. I am running performance friction pads and we properly bed them in. My coach has better braking.
Thanks everyone for your advice. To keep everything simple I've decided to go with stock replacements.
Before I can drive my coach here in Aus it needs an inspection and I believe that will be easier to pass if it is unmodified.
thanks again
Gerry
Before I can drive my coach here in Aus it needs an inspection and I believe that will be easier to pass if it is unmodified.
thanks again
Gerry
Have you talked with Rob Mueller...also from Aus? He has lots of experience with these things. You might even get a ride in his new Mustang.
Gerry,
If you think you want the 80mm calipers, go ahead. As I understand it, they fit under the steel wheels with just a little grinding. That be the case, the inspector would have to both know GMCs real well and be interested to remove the wheel to tell if it was non-stock.
Matt
If you think you want the 80mm calipers, go ahead. As I understand it, they fit under the steel wheels with just a little grinding. That be the case, the inspector would have to both know GMCs real well and be interested to remove the wheel to tell if it was non-stock.
Matt
Just an update. After re-reading the advice here I went with the 80mm update. I spoke to a local in my state who went that way and has had a good result.
The parts are still on their way to Aus so it’ll be a while until I get them installed.
Thanks all.
The parts are still on their way to Aus so it’ll be a while until I get them installed.
Thanks all.
I'm in the process right now of installing the 80mm calipers. These photos and .pdf of installation steps were provided to me by Dave Lenzi (I'm hoping he doesn't mind me sharing the info). Notice that there are a few areas that need to be ground down on the caliper and the knuckle. There may be some slight modifications needed to your pads, depending upon the pads. There may also be some slight modifications needed to the hose, to help with mounting the hose. Dave Lenzi modifies the brake hoses to fit/mount better, and is a virtual treasure trove of information on these GMC's. I also went with one of his sensitized brake booster and machined to fit master cylinders, along with a vacuum booster, and front brake lines for the 80mm calipers. I sourced the calipers locally. The booster and master cylinder were quick and easy installs, but I haven't had a chance to work on the installing the vacuum booster or the 80 mm calipers yet.
Attachments
Thanks for that. Those pics and PDF will be very useful. Especially showing where to cut and grind.
