Question: I have a relatively stock '78 Chevy K-10 pickup. It has a 4-inch suspension lift and a 2-inch body lift so I can fit my 35-inch mudders under it without cutting anything. It has an NP205 transfer case, and the problem is, the last couple times I have put my truck into an extreme situation (steep hillclimb up mud and grass, loaded with 1,400 pounds of dirt, or just trying to get over a few boulders), the front wheel or wheels begin to spin and the back end just sits there, as if it is somehow in front-wheel drive. Two-wheel drive works fine, and it even drives in four-wheel drive in an easy situation. But under stress, all of the power goes directly to the front wheels. Where do I begin diagnosing my problems?
Answer: There could be a couple of reasons why your transfer case won't send power to the rear wheels.
First off, are you sure it's a 205 and not a 203? The trucks built in 1978 had an automatic transmission with the 203 and no front locking hubs. The 205s came with a standard transmission. Or, it could be, now that your truck is approaching 30 years old, someone before you bought the truck and could have done a transfer-case swap.
The 203 is a full-time unit with a 2-Hi, 4-Hi, 4-Hi Lock, and 4-Lo Lock position indicated on the shifter. It's possible that if someone had swapped out a 203 in place of the original 205, then the shifter would not show this pattern. When the 203 is not in a "Lock" position, power will be routed to the wheel that has the least traction. In the instance you described, where you would be going up a muddy hill with a load in the back, the front would have the least amount of traction and you would notice that the left front was spinning.
The 203 is a physically larger case than the 205, but if you don't have the two cases to compare against each other you may not be able to tell the difference. The 205s do have an I.D. tag on the front of the case, but it's difficult to read when it is in the truck and covered with grime. The easiest way I have found to tell the difference is the small idler-shaft plate that is held on with three bolts on the back of the case between the rear output housing and the large aluminum front output bearing retainer. Only the 205 has this cover plate.
Now there is another possibility. According to NP205 guru Stephen Watson of Off Road Design, (www.offroaddesign.com), if there is a lot of wear in the shift rails as well as the internals of the transfer case, it just is possible that when shifting into low-range four-wheel drive that the shifter is not allowing the rear drive gear to properly engage. It's rare, but he has seen it happen. Again, this may be possible because of the age of the truck and especially so if there are a lot of miles without a rebuild.
Question: I own an '84 GMC truck that I built up with a Cummins diesel and a divorced NP205. The brakes are vacuum-assisted, and I used the stock 1/2-ton Chevy booster and am using the vacuum pump off the diesel engine. I don't have a strong pedal-instead, it feels spongy. I have brand-new calipers in the front and new wheel cylinders in the back, and the rubber lines are new. I have tried three different master cylinders.
How much vacuum does it need to work properly? Is the diesel engine pump providing the necessary amount of vacuum?
Answer: To start with, the first thing I would do is get a vacuum gauge and check the pull of the vacuum pump. If it's in good condition, it should put out some where between, say, 17 to 19 inches of vacuum. In reality, your power-brake booster should work just fine with 14 inches and most likely down to 12, and in some instances 9 or so.
A bad master cylinder or even a bad power brake unit will generally not give you a "spongy" feel to the brake pedal. This usually comes from air trapped in the line. I will assume that you did the proper bench bleeding as per the instructions that came with the master cylinder. It's not uncommon to have air trapped within the system some place, especially in the calipers.
I have tried all sorts of different ways to bleed brakes, but my favorite way is to attach a clear hose to the bleeder screw and run it into a container. Have someone push down on the brake pedal slowly while you open the bleeder screw just enough to let the fluid flow. Loosen the screw too much, and you take the chance of air leaking around the threads and giving you a false impression of never being able to get all the air out of the caliper or wheel cylinder. With the pedal on the floor, close the bleeder screw. Have the helper raise the pedal and count to 10 before pushing down again. This gives the system a chance to refill. Do this until you are positive there is no air in the system. This is the nice thing about the clear line as you can see any air bubbles coming out. You may have to refill the master cylinder several times. You never want to empty the master cylinder of fluid as then you're in for a long time of bleeding. Oh, and don't leave the cap off the master cylinder or the brake fluid can as it will pick up moisture from the air.
You may want to tap the calipers with a soft hammer as you're bleeding them to release any trapped air. Sometimes it is even necessary to take the caliper free from its mount and turn it so that the bleeder screw is at the highest point during the bleeding process. Sometimes it's necessary to find the combination valve and hold the metering valve pin in an upward position while bleeding, but not always. They make a special tool to do this with, but a pair of locking needle-nose pliers will work also.