Brake Bias Valve Misadventure

[RDKeysor]

A few weeks back I posted mention of a rear brake lockup solution published in the AH Club of America magazine Marque. The author had countered his problem by replacing the rear brake cylinders, .750" bore, as I recall, with Triumph cylinders of a similar construction but smaller bore size to reduce rear brake function. This raised the issue of whether a better approach might be the installation of a brake bias or compensator valve. I read up on the topic on line and learned that these valves are used on a variety of performance cars, but also to address the common problem of rear end over braking on cars fitted with front calipers and rear drums. I purchased a Dewhel valve on eBay and acquired the required fittings, in this case a 3/8" x 24 TPI male fitting that threads into the rear brake distribution block, the other end a 1/8" NPT male threaded end that goes into the outlet side of the valve. This seems to be the common thread for both the inlet and outlet sides of these valves. The valve inlet side also take a 1/8" male NPT fitting and a female 3/8" port that I connected to the flex hose that normally connects from the rigid brake line to the distribution block. No instructions came with the valve, but an arrow atop the knurled adjustment knob indicates thats more pressure will be allowed with the valve fully turned down clockwise. This presumably provides the maximum aperture, with the knob to be rotated counterclockwise to reduce rear brake force. With the valve installed (a couple of times), I was unable to get brake fluid to transit it. I don't think it even entered the valve. Had a helper (not the wife) pump the brake pedal, hold pressure on the brake pedal, etc., with the right side bleeder valve open. Nothing. Examined inside of the valve and nothing seems amiss, though I don't know how the valve actually works. I also consulted with others, including a brake specialty firm in CA. Concluding that the valve is not functioning, I have removed it and reattached the hose and bled the system. It works fine without the bias valve. I might add that though I bought the valve on eBay, and many people sell this item, eventually Amazon had UPS deliver a return label to ship it back to the seller (unfortunately it was still on the car and I didn't get the label). The real puzzle is that my eBay and Amazon accounts don't show that I bought the valve at either of those sites, which makes the return process kind of opaque. I intend to buy another similar valve because the braking issue remains, but I obviously need to know more on this topic.

 

[Brinkerhoff]

On a car with stock tires etc. if your rear brakes are "overworking" , the first place to check is the front brake function ,not the rear. In the 45 years I've been working on British cars I've NEVER seen the need to change the size of the rear wheel cylinders or install a compensator valve. If the rears lock up easily , the shoes are probably contaminated with brake fluid and should be changed or the front calipers are faulty. Its no different if one of the front wheels were locking up or pulling , its normally the other side that ISN't working. The stock system works perfectly fine and should need no modification.

 

[GregW]

No instructions came with the valve, but an arrow atop the knurled adjustment knob indicates thats more pressure will be allowed with the valve fully turned down clockwise. This presumably provides the maximum aperture, with the knob to be rotated counterclockwise to reduce rear brake force.

I've never installed a brake bias valve, but the valves I have used always close when you turn them clockwise.

 

[HealeyRick]

To start, I'm not a brake expert, but a couple of things come to mind from reading both your posts on the subject. If you were locking your rear brakes before the front, then I agree with Brinkerhoff. Something was wrong with your stock braking system that you didn't diagnose and repair before modifying it. Manufacturers don't build cars to lock the rears first, it causes them to have the rear end come around and cause nasty crashes. Adding the brake servo and increased size master cylinder created some more variables. A bigger master cylinder will give a harder pedal, but provide less pressure to the brake calipers and cylinders. I believe the reason a bigger master cylinder was specified for the BJ8 wasn't just because of the servo but because they changed over from Girling Type 14 front calipers to Type 16, which had larger diameter pistons than the 14s and would require additional brake fluid volume to actuate. In both the article from the Marque and SteveG's case they have changed front calipers, which you have not. The Marque calipers are 16Ps, which are the same as the Type 16, but with different mount ear spacing. Steve has got four pot Toyotas. You're still running the Type 14s from what I can tell. Before adding in a proportioning valve, which may just be a band aid, I'd check out the operation of the calipers and rear brakes as Brinkerhoff suggests. Then, I might try switching back to the original master cylinder to see how that affects things. If you do so, observe the amount of brake fluid that remains in the reservoir and that it doesn't bottom out. As Bob Spidell pointed out in the other thread, it really is whack-a-mole. When you change something from stock, unless it's completely the same as someone else has done successfully, there's no guarantee the changes you've made will work out the same. Above all, be careful and test whatever you do in a safe environment. Having bad brakes is bad.

 

[RDKeysor]

Thanks for the continuing education. I'm putting aside the brake bias valve exploration and am ordering new rear brake shoes, as there is in fact a bit of brake fluid contamination on one shoe. I am also enlightened on why Healey went to the larger master cylinder, a move that I mistakenly thought was prompted by the addition of the brake servo system. I'll see how the new brake shoes work before consider returning to the earlier brake master cylinder, which I still have on hand. Yikes!

 

[HealeyRick]

Note that I said the bigger master cylinder wasn't "just because" of the servo. Generally a servo will require a bigger bore cylinder than non-servoed brakes. But since you have the smaller brake caliper pistons, who knows what exactly the right size bore is? Figuring all this stuff out requires calculations by folks who went to a school with "Tech" somewhere in its title (not counting my favorite "Lowe Tech") But for us lesser beings, sometimes we need to swap parts in and out to see what works.

 

[maxwedge5281]

i have a dual gbraking system on both of my big healeys. i use a wilwood 1 inch bore master cylinder with dual circuits. i also have a porportioning valve on the line to the rear brakes. turning the knob clockwise reduces the amount of pressure to the rear drum brakes. i have four piston front calipers on my bj7 and regular two piston front calipers on the bn6. the one inch bore does create the need for heavier force to brake the car but certainlly not unreasonable.

 

[Keoke]

I'm putting aside the brake bias valve exploration and am ordering new rear brake shoe.

GOOD:
You might also want to send the new shoes and drums out for turning to insure concentricity and also order riveted shoe lining.

 

[John Turney]

Also, if you haven't done so already, when you have those brake drums off for turning, have them checked for balance. It may make your ride a lot smoother.

 

[HealeyRick]

Here's some more stuff to consider. If your drums aren't scored, there's really no reason to send them out for turning. Just use lots of Brake-Kleen and make sure they don't have any oil or grease on them. As John points out, having them balanced will help with scuttle shake (Hendrix Wire Wheel will do it if you have no one local). If you have to get your drums turned, the new brake shoes may not have enough lining material to make good contact with the drums that are now bigger. This is where Keoke's suggestion of getting thicker linings riveted on comes in. This stuff used to be common when all cars had drums and if you could find a place with a brake arcing machine, so much the better. But since the brake arcing process released asbestos into the air, product liability suits have caused most of them to be scrapped at the insistence of the garages' insurance carriers.

 

[Keoke]

WELL:
Riveted linings run cooler and tend not to squeal. Post OEM lining is thicker and may require arching. Most brake parts supply houses have access to certified safe arching facilities but no longer perform it in house.

 

[Bob_Spidell]

FWIW, in California, at least, no legitimate shop will turn our drums, since there are no minimum thickness specs to be found.

 

[RDKeysor]

My brake drums appear to have small balance weights welded on them. Can't recall if one or both have one or more weights, but the important thing is that this car runs down the road without a bit of cowl shake or vibration. It's so dang smooth that I am hesitant to replace the now somewhat dated tires. Interestingly, I personally replaced three broken spokes on a front wheel without any truing effort. Just snugged them. This, of course, required removal of the tire and the loosening of some adjacent spokes. I'm also a bike fanatic and own a truing stand and dishing tool, but truing a car wheel/tire looked pretty intimidating to me. Fortunately, I didn't have. I had occasion to visit to Dayton Wire Wheel circa 1964 to have broken spokes replaced on my BT7, which the PO, a law student at the U of Cincinnati, had equipped and drag raced on the track. The proprietor at Dayton Wire Wheel personally cut out the broken spokes, trued the wheels and then hand painted the spokes. He offered to sell me the Dayton wire wheel substitutes, saying that the Healey wheels were not well designed. I didn't have any money back then--a reporter for the Dayton Daily News--so I made do with the original wheels. My recollection was that the wheel firm was a relatively small operation located in or near Xenia, not Dayton, back in the day.

 

[RDKeysor]

I started this thread and hope I am finishing it. Received and mounted new brake shoes over the weekend. Very carefully cleaned the inside of the drums, which appeared in perfect condition. There are weights on these drums indicating someone has balanced them. The left side shoes had some brake fluid contamination, and that was surely the cause of my brake lockup. The shoes removed were riveted and the material looked much like tan hardboard of compressed leather. The new shoes are attached with some adhesive and have a black aggressive look with some surface porosity. While I haven't had time to break them in by doing a series of hard stops, I got no brake lockup whatsoever and no directional pull on several hard stops. I was in denial about the negastsisve influence of brake fluid that had gotten into this drum from a very small leak in the LR wheel cylinder. It didn't leak enough to be visible on the backing plate, etc. However, there was some fluid on the back of the wheel cylinder. There was also a bit of material on the inside of the drum, this all discovered when we replaced my differential at a Healey club tech session a month or so ago. I replaced the wheel cylinders after that discovery, but simply cleaned off the shoes with acetone. This was followed by my brake bias valve misadventure. Happy that I got my money back on that purchase. So, the lesson here, as suggested by some forum members, is to replace contaminated brake shoes, no matter how good they look. Thanks to those who contributed to what was an educational exchange from which I hope others also benefited. This is the forum at its best!