Low Brake Pedal

While waiting for a call back from telriv, I wanted other opinions. Telriv knows his stuff and will steer me right, but I still want to hear from you guys.

69 Riviera.

Front drum to disc conversion with Scarebird brackets.
Calipers from 2000 Chevy Astro van with AWD.
69 Riv disc master cyl with large reservoir in front.
69 Skylark "hold off valve".
All new rear brake components (had a bad wheel cyl).
New brake hoses front and rear.
Fluid is full.

The master is the correct one with the shallow cup on the plunger. There is no rod adjustment on this setup.

Vacuum holds fine in booster.

As soon as I did the conversion, pedal height was on the floor. Converting to the disc master cyl helped, but if I hold the brakes on full and accelerate, I am unable to hold the car back from rolling. Pedal travel is very long.

When I bled the brakes after installing the hold off valve (shop manual calls it a metering valve), I just had the brakes pumped and I cracked the bleeder until all air was purged from lines. But the shop manual says I need to depress the valve on the back of the hold off valve while bleeding.

Is this absolutely necessary?

What am I missing?

I really don't want to put an in-line adjustable proportioning valve in. I had one in for a couple of weeks with little improvement.

Anyone have any ideas?

 

Here is what I will do on Sunday when I get the chance:

Apply the parking brake and see if I get more pedal.

Pinch off rear brake hose and see if I get better pedal.

Can anyone tell me: if I didn't get all the air out of the master cylinder when I bench bled it, would the couple of quarts of fluid I pumped through the system while recently bleeding the brakes act as a final "bench bleed"? Or do I need to pull the lines and bleed the master again?

 

Thanks Wiz. But how will preloading (or prestarting) the piston inside the master cylinder make my brakes apply more/earlier?

Thanks again!

 

if you think booster...also make sure the correct length pin is in place ...also make sure your rears are adjusted up a bit more than usual ...

 

I"ve done a lot of brakes and sometimes air gets caught up in the metering valve. The way to cure it is to stomp on the brake pedal like an emergency stop. Then bleed the bleeders in sequence. A lot of people don't know to start with the bleeder furthest away from the master cylinder. Right rear, then left rear, then right front, then left front. Stomping the brake pedal pushes air out of little corners that normal bleeding doesn't. So even if you didn't correctly bench bleed the master, the air bubbles will be forced out of their hiding spots. I hope this helps.

-Josh

 

Low pedal and fading pedal can be caused by not getting all the air out of the master cylinder. When you bench bleed the master cylinder You "MUST" submerge the plastic bleed lines in the reservoir and pump the snot out of the master cylinder until no more air is in the lines.

If you didn't do this you will have awful brakes. :TU:

 

Thanks Josh. I'll do that.

I put the emergency brake on and the pedal didn't change.

I will pinch off the lines and inspect everything, especially the front calipers.

 

After bleeding over a quart of fluid out of the master, I'm pretty sure it accomplished the same job as bench bleeding.

I'll report back.

 

You shouldn't need a quart.. Only need to fill the reservoirs once.

Like I said the bleed tubes must be bent over the top and submerged in the reservoirs so they can suck fluid back into the master cylinder when it springs back from the pumping. If you are just trying to push it through the brake lines it won't work. :TU:

 

Are you sure? Bleeding and bleeding at each wheel won't have the same effect as bench bleeding?

So I guess I'll bend up some lines and bench bleed the master while it is still in the car. I'll jack up the rear to level out the master cylinder and make sure the lines curve up and back into the depths of each reservoir of the mc.

None of the master cylinders I've installed came with lines for bench bleeding.

Is it too late to even save this mc? In other words, will using it for a year without properly bench bleeding it hurt it at all? A new master is cheap, so if I have to I'll replace this master.

 

Won't hurt it one bit. I made the same mistake when putting my car together. It had been so long since I worked at my service station.

I even tried my brother's power bleeder after driving it to Batavia and back and it still didn't get all the air out. I finally asked him why the pedal sucked and he no more than got the words "bench bleed" out of his mouth when I did thisou: Then I put it on the bench and bled it with the lines submerged huge bubbles came out. They get trapped in there somehow.:idea2:

 

I've not bench bled at times, and not had a problem after bleeding the brakes like crazy. Now, though, I make sure to bench bleed just in case. Guess I'm getting old.

 

When I added the factory hold-off (metering) valve, I had to bend a new line that runs from the hold-off valve down to the junction block. The factory line was 3/16" but the fitting that met the master cylinder was actually the size of a 1/4" line. So I used 1/4" line and had to use a reducer to get it to thread into the junction block as it is a normal 3/16" fitting.

I will replace that 1/4" line I bent up with a 3/16" line and use a reducer at the hold-off valve to go from the 1/4" fitting down to the 3/16" fitting on the 3/16" line. In theory, I'm told, the extra diameter of the short line I used should not make a difference, but the Buick engineers deemed it necessary to use 3/16" line from the master to the junction block, so I will as well.

So I'll bench bleed it (while in the car - I have 1/4" lines I'll bend up to be bleeders) and change that line out and report back.

I do not know what diameter that line would be from the factory on a disc brake car. I'll see if Jason can tell me.

 

I changed the 1/4" line back to a 3/16" and bench bled the master cylinder. It stopps pretty nice now. Still not a high enough pedal for me.

I still have to push the brake pedal through the floor to keep the car from creeping forward under mild acceleration.

This winter I may put a new booster in and see if I can do anything with the adjustable rod.