My master cylinder and rear wheel cylinders are leaking, so i'm replacing them this weekend. I have heard (and read) many people having problems with the proportioning valve after doing something like this. My light isn't on, and i'd like to keep it that way. I was just going to list how I plan to do things, and ask if anything raises a red flag.


1. hand pump fluid out of old MC
2. bench bleed new master cylinder
3. install new MC. Use new pushrod? (manual brakes)
4. install new rear cylinders
5. re-bleed MC
6. bleed each wheel starting with the furthest from MC
7. mash brakes to re-center proportioning valve?
8. test drive

 

i'm pretty sure you have to hold the button on the proportioning valve to bleed the rear brakes.

Bleed ALL brakes. Pass Rear, Driver Rear, Pass front, driver front.

Power bleeding does wonders!

 

I have replaced just about everything on my brake system EXCEPT the proportioning valve. I have never touched it and (knock on wood) have had no issues while bleeding any part of the system. Usually, the toughest part that you are looking at would be removing the hard brake lines from the wheel cylinders without damaging them. The nut will rust to the line and twist right off. Trust me, I know!

Good Luck!

 

I quite honestly have never touched the proportioning valve when bleeding my brakes but
I decided to look it up in the 1981 factory service manual and, whaddya know, they say it
needs touching while bleeding the brakes.

It doesn't say *only* for the rear brake, and what needs to be done is dependent on vehicle:

 

You forgot step 2A- adjust pushrod to spec. (just to make sure it has play at the pedal). I don't remember what the spec is, but you need some play in the pedal for the m/c to work right with the booster.

Power bleeding is the easiest way, and a Motive Power bleeder can be had for less than $50. I fabbed up an adapter in about ten minutes using a scrap of bar stock and a piece of rubber.

 

Thats one of the biggest questions I have, I don't have power brakes, so no booster.

Do I still need to adjust the the rod?

The new (rebuilt) MC came with a new rod, should I use it or reuse the old one?

I noticed on the end of the new rod that there is a groove in the round part of the rod, should there be some king of bushing or something in there (it didn't come with one).

 

Unless the rod that came with it was exactly like the old one, I'd use the old one.
You still need to check that the pedal has enough play. I doubt the amount is critical so long as it's more than 1/4 inch and less than 3/4 inch, but I'd want you to look up the spec. You just don't want the rod to have NO clearance or too much clearance.
Too much=the brake pedal won't have full travel, Not enough=the master won't push the right amount of fluid.
Neither situation is common, but it pays to check.

 

Thanks, i'll use the old one then. I'll also ignore the proportioning valve, unless I run into one of the situations described. I assume the rod clearance is just how far down the pedal travels before the rod begins to move towards the cab? And power bleeding is just bleeding with the engine running, so that it speeds the proceedure up?

 

heckfire, I have a 1990 M/C and booster on my 81....its all doable with minimal effort..

 

Power (AKA pressure) bleeding is a method of forcing fluid through the hydraulic system
under some sort of pressure (generally, air) from the top. I see a Motive product talked
about on here but have no experience with it myself.

Pressure Brake Bleeder

Some people have luck with what's known as vacuum bleeding, where a vacuum is applied
at the bleeder screws to draw fluid through the system. I recently got one from Harbor
Freight based on a recommendation of a friend of a friend but I'm not too awful impressed
by it, the clear tubing connecting the bleeder screws & waste fluid containment vessel
never seems to totally fill with fluid so I never get the warm, fuzzy feeling that there is no
air being let back up into the system.

Brake Fluid Bleeder