I skimmed thru this and your other brake post
https://www.ford-trucks.com/forums/1...d-dog-and.html
Originally Posted by
F-250 restorer
Into the world of hydroboosters I go. What a day. I took off the hb and made an extra spacer after I disconnected the brake pedal arm and pulled out another 1/2". So I moved the unit that distance from the firewall. I also went to junkyard and took apart about 4 units and took the rods that connect hb with master, and a larger spring for the return of the b.pedal. Since I took so many rods I played with lengths.
I also bought two 75 chebby k20 4x front calipers for $17 ea. at AZ. I thought the Caddy calipers were big! These things look huge. I'm giving up on the Caddy ebrake calipers. I wanted to solve the problem of my r.rear dragging. I can't wait to try them out. When I have the $ I'll invest in a set of the TSM custom ebrake calipers for this truck. But until then, these chebby cals will lock up a set of 38" tires!!
I thought that while I was back there I'd replace my leaking l axle seal. Heck, since I have a spare set, I'm going to replace them both.
I'm hoping there won't be any more episodes of the self engaging brakes. This morning they were locking so bad I could not drive, and had to change the hb master rod length just to get it home. And since it didn't act up all that way, I knew I had something. It was rod length, and, hopefully, not p/s fluid contamination damage.
To the poster who mentioned that residual valves are only used when the master is lower than the calipers: That is not true. A 2lb residual is commonly used in disc brake systems. With these modern systems, however, it is usually IN the master. ABS brakes, in Orange, CA, installs them in all their systems.
First, I’m glad you went ahead and checked your master cyl rod length.
I guess you are referring to me and
fordman75 concerning the residual valves.
It’s not common at all to have a residual valve in a 4 wheel disc system. In fact the manufacturers have been going the other way for some time now by using what is commonly known as “quick take up” master cylinders and “low drag” calipers primarily for gas mileage. Here is a link explaining how the piston groove is cut differently so the pad rubs the disc less than a standard caliper.
You had mentioned in your qjet thread that you were after increased mileage.
Using a residual valve would be adding pressure and increasing the brake pad drag, reducing mileage.
I don’t know what master or HB model you used, but most of the HB units will accept a wide variety of different masters with different bore sizes etc and you can just pull out the residual from the rear port if it happens to have one.
Here are some links that explains residual valves and disc systems.
If you experience a brake lock up after a few applications of the brake pedal, it is directly related to a residual valve retaining the brake fluid within the lines and not allowing the fluid to flow back to the master cylinder. The problem is either the wrong residual valve being used, a drum brake master cylinder being used on disc brake calipers, a inline residual valve plumbed in to the brake system with a built in residual valve in the master cylinder or a defective residual valve.
In a stock system, the master cylinder is mounted high on the firewall. The gravity of the fluid in the master will cause 1-2 pounds of natural residual pressure, which is sufficient to maintain constant pedal height in a disc brake. It is for this reason, in most cases, a residual valve is not used with a disc brake. There is one very notable exception. This is when the master cylinder is installed lower than the caliper or drum wheel cylinder. In this case, the fluid would want to return to the master cylinder by flowing downhill like a river. The result would be a low pedal or even no pedal at all! To stop fluid rollback, we would want a 2 pound residual valve, which is just sufficient to stop rollback, but not enough to cause harmful brake application.
The Brake Man.com - ValveTechI
Typical Brake System Configurations
Brake Article© by Dean Oshiro
You can get the general idea from these links.