[Big D]

Hi all,

The last few weeks have involved more time spent on the brakes. If I had a dollar for every hour I have spent on the brakes, I would have quite a pile of cash built up now. This is a bit of a long story but it does show the various things I have tried to get these brakes working. I’m not there yet but I’m sure there will be some out there that can benefit from my experiences so far below.

You will recall the problem I was having was that when applying the brakes, the pedal basically went to the floor. It had a spongey feel to it and it just didn’t seem to want to ‘pump up’ to give anything like a firm pedal. From what I could see it appeared to be due to air being trapped in the wheel cylinders since the change to the Raybestos type pistons and full cups, which was something I had seen others having problems with.

My retired mechanic friend Brian spent a fair bit of time with me helping with different ideas about how to solve the problem. We spent a day trying different things and came away completely convinced there was nothing else wrong with the brake system, aside from the wheel cylinders. When clamping off the rubber hoses going to each of the axles, a firm pedal is suddenly restored. Releasing the clamp on each axle produces a proportionate amount of ‘softness’ to the pedal.

We tried a number of tricks to get the air out of the wheel cylinders, including using a pressure bleeder, a vacuum pump, a combination of both pressure bleeder and vacuum pump, bleeding both wheel cylinders at the same time, a combination of pressure bleeder and someone working the brake pedal, all without success.

I read a lot about the problems others had found. I saw what they had tried and it seemed to be everything that I had already done. I even removed the check valve in the master cylinder as I’d read that with my type of Hydrovac with its own built in check valve, you didn’t need one in master cylinder as well. Not that I really expected any improvement, but that made no difference. It was suggested to me that the hydrovac might have been creating some sort of vacuum effect with the wheel cylinders, so I built a small line to bypass the hydrovac. Again, I wasn’t expecting it to be that based on my fault finding and I wasn’t surprised when that also had no effect. That darned air just didn’t seem to want to come out of the wheel cylinders.

I started some ‘blue sky thinking’ trying to come up with novel ideas on how to attack the problem, including:

• Reversing or driving the vehicle up a steep slope like a steep ramp, blocking the wheels and bleeding on the angle
• Driving one side of the vehicle up a ramp and then bleeding one side of the vehicle
• Installing the brake backing plates rotated 90 degrees, so that the wheel cylinders were positioned horizontally (why wasn’t the M8/M20 designed like that?)
• Removing the wheel cylinder caps and compressing the pistons just a little with welders finger type vise-grips while bleeding
• Bench bleeding the wheel cylinders prior to installing them on the backing plate (how?)

It occurred to me that Tee connections on the brake backing plate should have been designed with a bleed screw built into it. The Tee connection is the high point on the wheel and it made sense, but I had never heard of anyone trying this. The Tee connection is used on all wheels but when it is fitted to the rear and intermediate axles, there is a spare 7/16-20 port which would normally be plugged. When the Tees are used on the front axle the flexible hoses go into that 7/16-20 port and so the 7/16-24 port below is plugged.

I got my hands on some brake bleeder repair kits which have a straight 7/16-20 thread. I could get some with a 1/8 NPT but these did not fit well enough and I couldn’t source any with a 7/16-24 thread.

I fitted the 7/16-20 bleeders to the Tees on the intermediate and rear axles with some good thread sealant in the port that would normally take the flexible hose if the Tee was mounted on the front wheels. Although you are bleeding at the input to the wheel cylinders, I did get some air out when I bled the system again, but not enough to make any appreciable difference to the brake pedal.

I had read about reverse bleeding the system and after talking to a couple of other people, I figured that if I could get some fluid pressure fed into the wheel cylinders then I might be able to extract that trapped air out through the bleeders I had fitted to the Tee connections on the rear and intermediate axles.

I left the pressure bleeder cap fitted with a length of plastic tubing on the master cylinder. I ran this tubing to a jar thinking this would catch any overflow from the master cylinder that was pushed through the system by the reverse bleeding.

I started on the rear axle and clamped off the rubber brake pipe to the intermediate and front axles so as to isolate them. I connected a smaller piece of plastic tubing to the pressure bleeder and connected it to the wheel cylinder bleeder screw. I connected my bleeding jar to the bleeder nipple I had fitted to the Tee connection. I only used a couple of pounds pressure, but it was a messy job as the fluid tends to want to come out of the threads on the bleeder screw. It appeared to work though and was pushing fluid and a bit of air up to the Tee connection and out into my bleeding jar. I periodically checked the jar connected to the master cylinder but this didn’t show any signs of fluid coming through that far. It looked like the fluid and air went straight to the bleeder nipple on the Tee connection which was what I was after.

I did both rear and intermediate axles in this way. The bleeder screw I installed on the Tee connections really does make this possible. The front axle wasn’t so straight forward and with it not having a bleeder I fed the fluid through the bleeder screws on the wheel cylinders and cracked the line connections to the Tee while I bled. I wasn’t sure this was that successful.

After all this reverse bleeding, I tested the pedal and found some improvement. I still had a pedal that went to the floor but after about three pushes it started to firm up and it got to the point where I had a spongey pedal for about half of its travel before it felt nice and hard.

I decided to try another plan that I came up with and which I hadn’t heard of anyone else trying. As I said before, I’m not sure why these wheel cylinders were vertically mounted. It seems they would do the same job being horizontally mounted and be a darned sight easier to bleed Anyway, I guess the designers had their reasons…

I removed the wheels and hubs from the two rear wheels. I also removed the guards over the lines on the axle and unbolted the Y connection on the axle. I then unbolted the brake backing plate from the axle and punched all the studs out. With some careful jiggling you can actually rotate the backing plates a fair bit to get the wheel cylinders in a better position to bleed.

I rotated the backing plates on both sides to get the first wheel cylinder I was going to bleed into the 11 o’clock position. The flexible rubber hose to the axle Y connection allows this movement and with the bleeder on the outside, this then becomes the high point on the wheel cylinder.
I pumped up the pressure bleeder to about 5 pounds and released the bleeder screw on that cylinder. It was literally a ‘holy shit’ moment as air spewed out into the jar like a bullet out of a gun. Once I’d done that cylinder I rotated the backing plate to get the other wheel cylinder into the 1 o’clock position. Again, heaps of air came out into the bleeding jar.

I repeated the process on the wheel on the other side of the axle and had exactly the same result. Air literally flew out of the bleeder into the bleeding jar. I never thought I would be so happy to see air in a brake system. It just goes to show how much air is trapped under those full cups when the wheel cylinders are in that vertical position.

Encouraged, I went and tried the brake pedal and I could feel an immediate difference. The pedal did not immediately go to the floor so I knew I was on the right track.

I repeated the process on the intermediate axle and again had good results with lots of air coming out of the wheel cylinders.

The front axle was even easier with having the flexible hoses and they allowed me to rotate the backing plates a fair distance around, almost to the 12 o’clock position. Again, lots of air came out of these cylinders.
After I finished bleeding them, I tested the brake pedal again. I now have some semblance of a brake pedal. It is a bit spongey on the first couple of applications but firms up after that. I still need to adjust the brake shoes after having the wheels apart and I believe this will make a difference to the feel of the pedal.

For the first time, I can hear the master cylinder operating as it should. However, when I apply the brakes I can hear a ‘whooshing’ sound in the line on the right side of the hull, where the joiners in the three lines are under the metal cover. I can’t see any reason why there would be noise at that point. Perhaps there is an airlock in the line there somewhere but that is what I am thinking. I am also finding the pedal is still 'following the foot' a bit still so that might suggest there is still air somewhere and contributing to the softish pedal so there is still more fine-tuning required. I will pressure bleed again using the bleeder block at the high point at the back and I’ll give that line a bit of a rattle/tap as I do it.

For anyone else making the change to these Raybestos piston setups I recommend you add in these bleeders. If the input lines to the axles are broken then you will need a way to get the air out before it gets to the wheel cylinders. You do not want the air going down into those cylinders and these bleeders on the Tees seem like the ideal way to do that. If you are careful in bleeding you should be able to get the air out of the bleeder before it gets pushed down into the cylinders.

I have been thinking about a way of adding bleeder screws to the Tees on the front axles and have attached a photo of what I have come up with. It simply involves adding a second Tee connection, fed by a fixed line from the lower one, with a bleeder screw in the end. I might use a bit of epoxy glue just to hold the second Tee to the first one. Someone else might have a better idea but this doesn’t involve any machining or making up of new parts.

As I say, someone may get some ideas for their setups from my experiences. If I was doing it all over again, I would fit the bleeders, carefully bleed at the bleeder screws on the Tee connections. Then rotate the backing plate to get the air cylinders as high as possible, and bleed them from there. That should allow them to full with fluid and get all the air out of them. Then I’d return the backing plate to the correct position and put the rest of the hub assembly together.

Anyway, that is quite enough for this post. Until next time…..