brake issues built big block. need help

[70Monterrs]

I have a big block chevy 402 that I built with a 278 XE cam from comp cams, edelbrock performer RPM heads and intake, holley hooker headers, roller rockers, demon 750 carb etc. The problem is that I have barely enough brakes to stop. they used to be better so i think something else went out, we put a vacume can in the engine bay to improve them and a new master cylnder a few years back, I am curious if this could be the proportioning valve or distribution block which is near the headers. Has anyone ever had this problem? My second issue is that the TH400 trans with floor shifter never would stay in first to race. it shifts on its own to second sometimes. We literally just rebuilt it with a shift kit and 2500 stall and new case etc and it still does it, I thought originally it was valve body but changing it didnt help. is it true that a TH400 vacume modulater is adjustable? thanks

[CrazyDavey]

Michael the is an adjustable modulator, most tranny have one. There is a little screw inside the main body, turn it clockwise to move the shifts up and counter to move the shifts down. If your's doesn't have one just go to any good parts house and get one to swap it out.

 

One thing I have found for both cars that help the brakes a bunch is getting a master cylinder with a smaller main piston. Most that you buy are 1 1/8" to 1", you have to look at various applications but you can get one that is 7/8" and it will change the braking rate a lot, at least that's what worked for me...Dave

[DavidTX]

I have been having the same problem but have been unable to even locate a Master Cylinder with a 1" bore that is compatible with front disc brakes.

Can you or anyone enlighten us on what application will work with either 1" or 7/8" bore?

[stangeba]

"My second issue is that the TH400 trans with floor shifter never would stay in first to race. it shifts on its own to second sometimes."

70monterrs

That's the way Chevrolet designed the THM400, to shift into second gear, at a higher RPM, (how much higher I don't know) even when it is locked in first, floor shift or column, doesn't matter. The THM350 is NOT built to do this so don't try it!

Bruce

[kc8oye]

Bruce, no GM transmission relies on engine RPM for shift points.

they are controlled 100% by transmission output shaft speed.

 

but otherwise, you are right, the TH700r4 and 400 both will force a 1-2 upshift above a certain ground speed regardless. This can often be aggravated by changing rear gear ratios.

 

there are a couple of solutions to this

1) remove weight from the transmission's governor's weights.. this will move ALL shift points up.. up and down shifts.

 

2) if this is undesireable.. it will require some modifications to the valve body. most quality shift kits correct this.. apparently the one used in your transmission doesn't.

 

3) go to a full-manual valve body and shift it yourself all the time.

[wallaby]

I had a TH400 that I routinely let upshift itself that way. It would hold on to first gear a little longer than if I had the gear selector in the 2nd gear position. The upshift was much more solid, and the output shaft speed equated to about 5500rpm engine speed when it would force its upshift. It was perfect I thought...all I had to do was put the selector in 1st, then hold the throttle down until it forced the upshift, and THEN move the selector to 2nd. There is no forced upshift from 2nd to 3rd.

The downside is that the trans will dive back down into 1st if you let off the gas...not so good if you have a forced upshift in the water box and then let up on the throttle. Just don't forget to move the selector out of 1st if the trans upshifts!

 

As for the vacuum modulator, it only adjusts part-throttle upshifts. If you find yourself shifting 1-2-3 before you cross a leisurely intersection for example, you can adjust an adjustable modulator to delay the shifts and let the engine rev more freely between upshifts, but it won't affect the full throttle shifting points.

 

Your brake problem seems to be the main focus of your post though.

The power brake booster relies on engine vacuum to provide the boost. If your engine idles rough enough with thet cam, it may just not have enough vacuum to operate the brake booster well. (Put a vacuum gauge on it and find out). Adding a vacuum can can only store vacuum, it doesn't add to it. If your engine only pulls 10 inches of vacuum, the can will only store 10 inches of vacuum. The advantage to a can is when you are decelerating..in that situation, your engine might pull up to 14 inches of vacuum for example...and the can will store that 14 inches...and so does the brake booster chamber. That added volume of the can might let you stab the brakes twice instead of once.

[DavidTX]

I have installed an electric vacuum pump and that doesn't seem to make any difference. Cannot lock up the brakes during an emergency stop. That can be very dangerous.

[wallaby]

I have installed an electric vacuum pump and that doesn't seem to make any difference. Cannot lock up the brakes during an emergency stop. That can be very dangerous.

I lived with the same problem for years, adn was begining to think it was normal for an old car. The thing that kept me persuing the issue was that my pedal just felt "funny".

I took the car in to a mechanic and he found issue with the rear brakes...some problem with how the parking brake was adjusted. (I wish I knew exactly what was wrong there). If I remember right, he said the E-brake was adjusted too tight, and then the star adjusters were adjusted loose to compensate, and the result was poor shoe contact to the drum. Whatever correction he made brought up my pedal height significantly.

But I was still having a problem with how the pedal felt: sometines it was high, but most of the time it traveled quite low before anything would happen. I was pretty sure air was in the system even though I had bled them numerous times. I looked real close at my hose connections and found a slow leak on my RF caliper where the brake hose attaches. it turned out that my brake hose had a "dink" in it and wasn't seating perfect against the copper compression washer that seals the union. It was BARELY seeping fluid out over time, but it was readily letting air IN.

Once I had that problem fixed, I pressure bled the system, and the difference was like night & day.

 

I made one of these for the bleeding. I used a heavy flat metal plate instead of the master cylinder lid with a flat rubber gasket, and clamped it to the master with a big c-clamp.

http://www.bmw-m.net/TechProc/bleeder.htm

[72 LS5]

I went through this weak braking issue a couple years ago - even converted the car to manual disc setup, then back to stock. I wish the details were still fresh in my head.

 

At any rate the first thing I would do is get the brake pressure gauge to see if you have enough pressure at the calipers and rear cylinders. Chances are you do not have enough vacuum, like its been mentioned. I tried the pump and was not happy with the noise and vibration. The smaller master sounds like a good solution too.

 

Bottom line for me is that I could do no better than the factory setup adjusted properly. My cam just ain't pullin enough vacuum. So my brakes still suck but I'll see if the new 502 with a conservative cam helps. Oh - and make sure your headers are not cooking your brake fluid!

 

On my 85 monte I went from a regular single diaphragm booster to a 86 dual diaphragm master and it helped tremdously! They are much more efficient.

[maddmann313]

did you hook this line back up???

 

 

 

it comes for the brake booster and hooks in to the original quadrajet on the base plate and there is no where to hook it up on any other carb...

 

you have to hook it up on the intake in a vacuum port!

 

I had this same problem i took my old jet off and replaced it with a holley and tried to dive it and the brakes were HARD.... Hope this helps!

[kc8oye]

there is always the hydraboost option.