Bad Brakes??

I recently put Speedway's GM disc brake kit on my 1953 Ford F100. I also installed a 7/8" bore Wilwood master cylinder under the floor with RPV's inline and an adjustable proportioning valve to the rear drums. I have a good bleed and good pedal, but the brakes are just not very good. They do not slow the truck down very fast and I cannot lock them up no matter how hard I stand on it. Any ideas where my problems may lie? Thanks guys.

 

Are the bleeder screws at the top of the calipers? How's the adjustment on the rear drums? Have you tried it with the proportioning valve backed off all the way?

Flatman

 

I had the same problem...never really got the brakes 100%.....I heard that the master cylinder should be above all the brake lines....I know someone on here will have the answer...I would also like to know....

 

Assuming you have a GOOD bleed, then I would wanna know what your pedal ratio is. Disc brakes require about DOUBLE the line pressure as drum brakes do - you need to design your system around that.

Which M/C are you using? I see it's Wilwood, but which one??? Does it have enough volume for your application???

Is the pedal soft and mushy or stiff as a board???

Which calipers are you running?? Can you see them "working" when you bleed the system??

 

I'll take a random stab at this. First lets assume that the residual line valves are in, pointed in the correct direction and installed 2lb front, 10 lb rear. Next junk that water faucet proportioning valve, this is a racecar band aid that only restricts line flow, without taking your system completely apart open it up full for no restriction. If you do get to much rear bias you can shut it down then. Carefully adjust the rear drum brakes so they evenly just have a audible shoe drag. Now the next issue is the very 7/8"small Wilwood master cylinder. 7/8" bore doesn't really displace enough fluid to fill the big GM calipers and from your description of brakes but no stopping power, this would indicate that you might be bottoming the cylinder before the brake cylinders are filled. To check this jack up the front end of your truck to lift off a tire. Have someone slowly step on the brakes and see how long you can rotate the wheel before it locks up and wont turn any more, note the position of the pedal travel. If it is way low its a displacement issue and you will need a 1" bore unit. If its high then you'll have to examine the pedal to master cylinder arm ratio. If you have homemade pedals this could be the problem, if you have stock pedals this should have the correct ratio so you should be ok. Try this first and them tell us what your results were.

 

<TABLE class=searchitem><TBODY><TR><TD class=image> </TD></TR><TR><TD class=title>Wilwood 260-9439 </TD></TR><TR><TD class=description>Aluminum Masle Cylinder 7/8 Tandem Chamber Standard Finish </TD></TR><TR><TD class=description></TD></TR><TR><TD class=partno>Item# 950-260-9439 </TD></TR></TBODY></TABLE>

Here's my master cylinder. With the size of the resevoir I wouldn't think it was a displacement issue, but I guess that could be the case. I'll get it up in the air this week and check it out. The bleeder screws are at the top of the caliper, the rear drums are adjusted. The proportioning valve is not open all the way, I will try that this weekend as well. Thanks guys, I'll keep you posted.

 

The resevoir size is not the issue, the 7/8s bore is. Most American disk brake systems use either a 1" or 1&1/8 bore cylinder. But again is your pedal linkage ratio correct ,not enought leverage will make for a hard pedal and not stopping worth a shit. Smaller bore cylinders give higher line pressures with a given input force compaired to larger bore cylinders. Have someone apply the brakes and measure the travel of the master cylinder pushrod . I would think you should see no more than .5 to .75 travel max.