brakes

hi i need help
I have a speedster with disc brakes in the front and drums at rear.
at high speed wanting to to stop is a bit like stopping a train on ice, as the front wheels lock up .Do i need more weight infront or can it be solved with an adjustment

Hmmm, this sounds like a proportioning valve issue........i.e. same amount of pressure is being applied to the front set up with discs as it was when there were drums?

I'm not 100% dialed up on what you guys have, but over here most guys change the master cylinder set-up once they go discs mounted up front.

Anyone else either confirm or deny this?

First make sure you have the rear drums adjusted properly --- they are NOT self adjusting like new vehicles. Weight in front always helps since front brakes do 70-80% of braking. A big lead battery in nose and a 16 gallon fuel tank help.

First, check to see that you hav rear brakes that are properly adjusted and working. Be sure to loosen the E brake cables at the handle before making a brake shoe adjustment then take up cable slack until you have a good E'brake handle with about 4 - 5 clicks.
Release the handle and have someone push on the brake pedal while you listen for the rear shoe movement if not, then check for fluid to the rear wheel cylinder by loosening a line at the cylinder one side at a time you should get a good squirt. Be sure to catch this fluid in a shop rag and not get it on any painted surface. Also, look the wheel cylinder piston movement.
If you do not have good flow, tighten the line and loosen the rear line at the master cylinder port, push on the brake pedal confirm you have fluid there. ~Alan

Gary: Most of the speedsters over here have later-model (think Super-beetle), dual circuit (front/rear) Brazilian master cylinders which are almost adequate for a Speedster, but typically need a little help. While a proportioning valve can be used, they are big (because of the housing and the knob) and, quite frankly, are not really needed on a Speedster unless you're racing it and may be changing brake pad/shoe compositions often and have a need to compensate for differing stopping coefficients caused by differing pads (or differing driver's tastes in multi-driver racing scenarios, like LeMans or Rolex long distance races).

So..... After you've adjusted the rear brake shoe adjusters and found that they were pretty much adjusted correctly in the first place and still you're getting far too much braking on the front and not enough on the rear, then get a 10 lb. residual valve and have a good brake shop install it anywhere along the length of the rear brake line BEFORE it gets to the "T" at the rear. It can go on at the master cylinder or anywhere along the line going to the rear (I like it about half way back, because it's easy to get at the brake liine and the seat hides the valve.

Get one of these: http://www.wilwood.com/MasterCylinders/MasterCylinderValves.aspx

Scroll down to the red, 10 lb. residual valve.

CB Performance has them, too, at higher prices.

BTW: ALL new cars with disk front and drum rear braking systems have these valves in their circuits. Some master cylinders even have them built in (like Honda).

Oh - I almost forgot.....WHY do you need this valve??

Because there are two pistons within your master cylinder (separate front and rear circuits) and they're both about equal in the amount of fluid that they move in a pedal stroke (and they were probably designed with drum brakes in mind). When you mix disk and drum brakes, even on different circuits connected to the same pedal stroke, you cause an imbalance in braking because disk brakes need to move far less than drum brakes and, therefore, need less fluid to actuate them. When you step on the pedal, the fronts engage first because, being disks, they need less pedal travel to engage so the rears are sort-of floating back there and not doing much. As you're about to run into the rear of that big SUV in front of you, you push much harder on the pedal but even that doesn't move much more fluid because the front circuit is pressing back and not allowing the pedal to move enough to engage the rear drums - your front brakes are doing all the work and your rear brakes are sitting there laughing as you hit the SUV.

The redisual valve keeps 10 pounds of pressure in the rear circuit, thereby keeping the shoes almost, but not quite, engaged. When you step on the pedal, both circuits move the same amount of fluid (not really, but close enough for this discussion) and then engage both the front and rear circuits at roughly the same time and with the same amount of force. Bingo! Both ends brake the car.

Hope this helps.

Gordon
The Speedstah Guy from Beaufort

Gordon, would our cars use the NA fittings as supplied by Wilwood or would we need metric?

Brian:

That only matters if you attach the add-on valve directly to the end of the brake line and use the existing (double-flare, metric) fitting. If you mount it there, you'll need another short length of metric, double-flare brake line to finish the connection.

Because of the nature of the fitting (you need a special, preferably hydraulic, flaring tool to get that double flare) I prefer to just cut the rear brake line somewhere along where the seat is, and then do a standard (American, single or trumpet) NA flare to match the residual valve seats and threads. I also have a hand tool that can make that type of flare at home, rather than pull out the line and take it somewhere to have it done.

Depending on how you do this and the length of the valve you purchase, you can either pull a bit of brake line through on one end to remove the slack, or simply cut out a short piece (an inch or two) and put the valve where the piece was removed. You have to be careful in how much you remove - make it too short and the lines won't reach any more!

Again, I like to put them beside the seat because you've got lots of line to work with, you can hide the valve easily beside the seat track AND there's a few places you can maneuver the line into to remove the slack. Once it's installed, you never have to mess with it (unless it leaks, of course).

gn

one bit of advice if you go with custom bake lines to an adjustable proportioning valve - make sure you you use bubble flairs, not SAE flairs on the ends.

after getting the lines made, installed, bled, it still leaked - so i ripped it all out and it's sitting on my workbench waiting for more time (and new ends).

while we're on the topic of brakes - i figured out how to bleed my brakes by myself in a matter of minutes. it took one extra reservoir cap, barbed fitting and some clear hose. i used my existing compressor with pressure regulator set to 7psi. worked like a HOT DAMN.

Flare pictorial:
http://www.fedhillusa.com/webnuts/common%20flares6.pdf

They do make adapters to go to dbl. bubble flare

I've gotta say, that I've made a LOT of brake lines with single flare connections and rarely get a leaker. In fact, the car club in Beaufort re-lined a classic Pontiac Parisiene and had a newbie do all of the flared ends and we didn't have a single leaker. And that was with a flaring tool from Harbor Freight! (we're still amazed with it all, to tell the truth).

Bubble flares are nice, they're what VW used, but they still are almost impossible to do with a hand flaring tool. So if you choose to go that route, take the lines to a brake shop and have them flare the ends for ya.

Thanks for the chart, Scott!

Great chart, it's now on my shop wall ~

You're welcome Gordon - (also see home page for tool directions and video - http://www.fedhillusa.com/ )