Front Discs will not engage

[Bare64]

Searched here, Googled, bleed and re-bleed the system, made sure proportional valve lines are correct (they are) and still I can't get the front disc's to pressure up, rear brakes work fine. One thing that keeps coming up is the heim rod end at the pedal so I extended it out as far as I felt safe doing so, only about 5 threads left. I am thinking that if I do the modification by moving the hole on the pedal up one inch that all I will accomplish is engaging the rear brakes quicker. Anyone know off the top of their head what the eyelet size and thread size of the joint? Any help or hints would be appreciated.

Andy

[blown240]

Do you have the correct master cylinder? The disc drum masters have a large side and a small side in the reservoir. Im pretty sure the Discs go to the large side, since it takes more fluid to get fill the calipers vs. the wheel cylinders.

Also, do you have residual valves installed?

[61_FL_Apache]

Check your rubber lines. I had them totally collapse internally and not let fluid pass easily.

[Bare64]

Originally Posted by blown240

Do you have the correct master cylinder? The disc drum masters have a large side and a small side in the reservoir. Im pretty sure the Discs go to the large side, since it takes more fluid to get fill the calipers vs. the wheel cylinders.

Also, do you have residual valves installed?

@ blown..........Correct master cylinder, large and small reservoirs along with a stock Chevrolet proportional valve. Why would I need a residual valve, the master cylinder is above on the firewall?
@ Apache....... no clue here, they seemed to have bleed easily enough. How does one tell if they are collapsed internally?

[blown240]

I thought that you still had to use residual valve even with a prop valve.

[Bare64]

Originally Posted by blown240

Do you have the correct master cylinder? The disc drum masters have a large side and a small side in the reservoir. Im pretty sure the Discs go to the large side, since it takes more fluid to get fill the calipers vs. the wheel cylinders.

Also, do you have residual valves installed?

Originally Posted by blown240

I thought that you still had to use residual valve even with a prop valve.

I think it should be fine with the master cylinder being above parallel to the brakes themselves.

One thing weird is the brake pedal feels fine until I start the truck, then it collapses to the floor.

[blown240]

I bet you still have some air in the system.

[geezer#99]

Did you change your pedal ratio?
Stock drums use a 6 to 1 ratio, power needs a 4.5 to 1 ratio.
You need to drill a new hole about 1 inch below the brake rod hole on your pedal.

[donthekawguy]

What geezer said and check you booster.

[luvbowties]

Originally Posted by Bare64

I think it should be fine with the master cylinder being above parallel to the brakes themselves.

One thing weird is the brake pedal feels fine until I start the truck, then it collapses to the floor.

You're right: no residuals needed. See below. "Top Ten Reasons For A Poor Brake Pedal" ESP. the 1st reason shown re: bleeders must point to 12:00 o'clock--which they won't do if left-and-right calipers are reversed. If so, you'll never get the air out of the front.

>> http://www.piratejack.net/index.php?...port&Itemid=20 <<

HTH,
Sam

[Bare64]

Originally Posted by luvbowties

You're right: no residuals needed. See below. "Top Ten Reasons For A Poor Brake Pedal" ESP. the 1st reason shown re: bleeders must point to 12:00 o'clock--which they won't do if left-and-right calipers are reversed. If so, you'll never get the air out of the front.

>> http://www.piratejack.net/index.php?...port&Itemid=20 <<

HTH,
Sam

They are more like at 1:00, going to bleed it again. If that does not work looks like another new booster, master cylinder is new and correct.

[w00dst0ck]

Originally Posted by blown240

Do you have the correct master cylinder? The disc drum masters have a large side and a small side in the reservoir. Im pretty sure the Discs go to the large side, since it takes more fluid to get fill the calipers vs. the wheel cylinders.

Also, do you have residual valves installed?

3/16 front
1/4 rear

Smaller lines to disc brakes.

[Mathew21]

Bare64 I'm sorry if I missed something here but did you just do an upgrade to disc brakes or is this a problem with a brake system that was previously working fine?

If you're a 100% sure you have no air in the system then it sounds like a bad master cylinder. I don't know if your master cylinder is a new or not. But if it is new does not mean it's good. I have a had bad "new master cylinder" I purchased from an autoparts store more than once.

Below is something I copied from another site. I hope it helps explains why I think it sounds like the master cylinder.

1) If the booster stops functioning, the main effect in an otherwise functioning brake system is that the pedal feels as though it is seized up and cannot be moved at all with normal foot pressures. If very heavy pressure is exerted, some small braking effect may be noticed.......very, very hard pedal is usually the result.

2) The master cylinder generally does not leak outside of itself, it leaks internally and the fluid travels back into the reservoir, so usually no fluid will be seen on or under it or lost if it's bad.

[64shortbox]

Bare64,
Sorry I didn't see your post earlier. Since you are using a factory GM proportioning valve - did you use that little tool to hold the proportioning valve centered when you bled the brakes? If not you need to remove (unscrew) the switch in the middle of the valve (the plastic thing that the brown wire hooks clips to that triggers the "BRAKE light on a stock GM vehicle). Once you remove that switch you can look down inside and see a slider with a grove or U cut in it. That grove must be held centered when bleeding the brakes or your brakes won't bleed properly. You can re-center the valve by reaching down with a small flat blade screwdriver. From what I understand if you loose say a front brake line the valve will shift to shut off the brake fluid to the front brakes and divert all fluid to your rear brakes. I'm attaching a picture of the little tool. It looks like it's anodized aluminum but it's just made out of plastic.
So remove the switch - center the valve & install the centering tool to hold the valve centered - bleed your front brakes - remove the centering tool - reinstall the - Brake light switch and you should be done. I bought my tool off eBay years ago but I'm sure Napa or another decent auto parts house would have one. Hope this info is of some help to you.

[w00dst0ck]

Interesting. I've never heard of having to do that.

[Bare64]

Originally Posted by 64shortbox

Bare64,
Sorry I didn't see your post earlier. Since you are using a factory GM proportioning valve - did you use that little tool to hold the proportioning valve centered when you bled the brakes? If not you need to remove (unscrew) the switch in the middle of the valve (the plastic thing that the brown wire hooks clips to that triggers the "BRAKE light on a stock GM vehicle). Once you remove that switch you can look down inside and see a slider with a grove or U cut in it. That grove must be held centered when bleeding the brakes or your brakes won't bleed properly. You can re-center the valve by reaching down with a small flat blade screwdriver. From what I understand if you loose say a front brake line the valve will shift to shut off the brake fluid to the front brakes and divert all fluid to your rear brakes. I'm attaching a picture of the little tool. It looks like it's anodized aluminum but it's just made out of plastic.
So remove the switch - center the valve & install the centering tool to hold the valve centered - bleed your front brakes - remove the centering tool - reinstall the - Brake light switch and you should be done. I bought my tool off eBay years ago but I'm sure Napa or another decent auto parts house would have one. Hope this info is of some help to you.

Thanks, never heard of this but I will purchase one before I bleed them again this weekend.

[Bare64]

Ok, looking down into the valve after removing the brake switch trigger I do not see any grooves only a flat rod. How do I move the rod without damaging it?

[Bare64]

Where did you find the tool shortbox? Even my master mechanic has never heard of nor seen one.

[Palf70Step]

Sounds like your rod is not centered, and until you get it centered they won't bleed right.
Pic is what the tool does once the valve is centered.


This may help to get them centered and bleed right
http://www.classicperform.com/TechBo...t.htm#testprop

[joedoh]

thank you for explaining why I couldnt bleed the brakes on a 1988 mazda pickup about 10 years ago. it had that same type of distribution block and the front discs would just not engage no matter how much I bled them. I ended up replacing the part.

this forum is awesome, I have lost track of how much I have learned here.

[Mathew21]

Originally Posted by Bare64

Where did you find the tool shortbox? Even my master mechanic has never heard of nor seen one.

They sell it on EBay but I'm not sure where you would find it locally.

http://www.ebay.com/itm/Combination-...-/160856168243

[Bare64]

Originally Posted by Mathew21

They sell it on EBay but I'm not sure where you would find it locally.

http://www.ebay.com/itm/Combination-...-/160856168243

Thanks, ordered one. Not a bad price at all, but I still have no clue as how to get the "groove" back to the center of the proportional valve. Anyone have an idea of how to accomplish this?

[luvbowties]

Originally Posted by Bare64

Thanks, ordered one. Not a bad price at all, but I still have no clue as how to get the "groove" back to the center of the proportional valve. Anyone have an idea of how to accomplish this?

You asked, "Anyone have an idea of how to accomplish this?"

An above site-- http://www.classicperform.com/TechBo...t.htm#testprop -- given by Palf70step said, modified by me: "5.The lines that are clear must be left open[BOTH YOUR REAR BLEEDERS] and the blocked ones should be closed to cause pressure to build up on that side. Be sure to use the standard bleeding procedures to prevent air from entering the system.
6.Slowly press the pedal with steady pressure[until pedal reaches floor--then tighten your rear bleeders & slowly let pedal up] a number of times until the light goes out[you prolly do not have a light to see--ok]**REMEMBER TO KEEP MC FULL OF FLUID DURING THIS TIME.**; this will center the differential valve. You may also hear a pop come from the proportioning valve. This is the metering valve returning to its equalized position. When the light goes out[again, you may not have one to see], close the bleeder screw. (See fig. below)."

I hope this works/helps. But proceed with caution, 'cause this is "only how I am reading the instructions"--which sounds logical to me. Kinda like when you press pedal with rear bleeders open, it's like you suddenly lost your rear brakes and the valve rushed to shut off the rears & make all pressure & fluid go to front ones. Sound logical to you?
Sam

[Bare64]

Originally Posted by luvbowties

You asked, "Anyone have an idea of how to accomplish this?"



I hope this works/helps. But proceed with caution, 'cause this is "only how I am reading the instructions"--which sounds logical to me. Kinda like when you press pedal with rear bleeders open, it's like you suddenly lost your rear brakes and the valve rushed to shut off the rears & make all pressure & fluid go to front ones. Sound logical to you?
Sam

Makes sense to me too, had not gotten that far before I ask my question.

[fleetsidelarry]

good info, saved to favorites. thanks to all