You have discovered the mass of a Newell. Stopping a 45,000+ pound vehicle takes a lot of space, especially on a downhill grade. The jake on a two-stroke Detroit doesn't have the braking force of a jake on a four-stroke Detroit and as soon as the rpms drop, it loses effectiveness quickly. As to locking up the wheels, I have never had a wheel lock up on me when stopping aggressively other than on gravel. As a matter of fact, the newer coaches have anti-lock brakes to make sure that they don't lock up. However, the brakes should be on the verge of locking up when you are standing on the brake pedal.
Since different people have different perceptions of stopping distances, I would suggest that if you haven't have the slack adjusters checked, this would be a good time to do so. I had Newell adjust mine about a year and a half ago and they all needed some tightening. You can adjust them yourself if you know how and have a way to block up the coach so it doesn't come down on you while you are under it. While you (or a mechanic) are down their, you can also check the thickness of the brake linings. It takes a long time to wear out the brake linings but it is a good idea to check (or have them checked) about once a year.
I presume that your air pressure was good (brake air pressure gauges in the 110-125 range) prior to the stop and that the pressure was still above 95-110 after the stop. Some people used to car hydraulic brakes make the mistake of pumping their brakes (although it certainly doesn't sound like you did) and find they run out of air very quickly. An air leak in the brake can ruin your entire day. Do you do the air brake test recommended for commercial trucks periodically? If you don't (or are not familar with the procedure) download a CDL manual from your state and go through the air brake testing procedure. It will give you confidence that the air is staying up and that the emergency spring brake will work if needed. Also, verify that the brake air pressure isn't dropping with the supply air pressure when you are not using the brakes. Tom and I have both had to replace the air check valves that were allowing air pressure from the twin brake tanks to bleed back into the supply tank. That is an easy test. Start the engine and get your air system up to the compressor's shutoff point (typically around 125-130 psi). Shut the engine down and without touching the brakes, see how fast the supply system drops. If no air is used (air doors, genset slide out, etc, the supply pressure should not drop more than 20 psi in about 40 minutes. HOWEVER, in that 40 minutes, the brake air pressure needles should have stayed virtually unchanged. When the check valves start malfunctioning, one or both brake pressure needles (there is a white one and a red one) will follow the supply gauge down and that is a very dangerous situation.
Since you are concerned about the stopping distance, I would suggest you have the wheels and drums pulled and see if the linings are glazed or the drums badly scored. New linings are cheap compared to not being able to stop in time.
I believe it is a good idea for everyone to find a safe location where they can get up to 50 mph or so and get down HARD on the brakes. You may be surprised who long it takes to stop 45,000+ pounds. You will also benefit from knowing what to expect and what it feels like to do a panic stop in a massive vehicle with air brakes.
This is a great topic and I really appreciate you bringing it up. I see many heavy coaches disregard the reduced truck speed limits on downhill grades. I personally think that is a bad decision. A Newell's weight and brakes are very similar to a large truck and if it isn't safe for them to go down a long 6% grade at more than 35-40 mph it isn't safe from us either.
I have found that manually downshifting when trying to stop quickly helps reduce stopping distances. The older HT-740 transmissions did not downshift quickly enough to keep the engine speed up for the jake to help out very much. You want to downshift as soon as the engine drops to about 1600 rpms. That will maximize the engine braking with breaking the engine.
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