Brake pedal goes almost to the floor before brakes engage

[Jim S]

I have completed the Turner Brake upgrade and all new drum brake parts on the rear.  Today I got my original booster rebuilt back from Studebaker Itnl and a new master cylinder installed. I bleed the system 3 times.  The brakes engage, however only until the pedal is almost at the floor!  I checked the measurement of the booster to master push rod and it is correct. Any thoughts welcome.

Edited Friday at 10:49 PM by Jim S

[Zedman]

Jim, what year is your Avanti?.... Does it have a proportioning/Combination Valve?   If it does, did you bleed your brakes using the centering tool made for this purpose?  If you don't use this tool to fix the position of the internal shuttle valve, half your system will be left unbled.

[Jim S]

It is a 72.  The brand X booster and master cylinder that was in there had a proportioning valve in line.  I used the same valve with the OEM booster and master cyl that I put back in. I bled the system first without the valve as I was told the car originally did not use one.  Second time around, I put the valve back in and re bled.  It made no difference. The front calipers are grabbing, but with the rear wheels off the ground, I put it in gear and applied the brakes.  The rear wheels kept moving. Also, I don't know about the centering tool for the valve, so of course I did not do that!

Edited Saturday at 01:25 PM by Jim S

[Zedman]

I think the use of a Combination/Proportioning valve in a disk+drum brake system is a worthy addition.  I hope I'm not telling you to 'suck eggs', but one purpose of the valve is to redirect fluid to the rear brakes to avoid locking up the front brakes. the other purpose is to sense an open circuit, and then direct all fluid to the effective brakes- front or rear. The tool is used when bleeding.... you remove the brake warning light switch from the valve and replace it with a rigid pin that prevents the shuttle valve moving to either side (trying to redirect flow one way or the other).  I had an obscure Kelsey-Hayes Valve on my 1981- I'm replacing it with a GM PV2 valve and its even a bolt-up fit with the old one. The valve came with the bleeder tool.  See this video for a clearer explanation. I'll bet this is what is causing your problem.

 

Edited Saturday at 02:12 PM by Zedman

[Jim S]

Great minds think alike.  I just watched the same video a little while ago! Thank you for your advice.  I ordered the tool from Amazon and it will arrive tomorrow.

[Adam DeRosa]

@Zedman, that's a great video - thanks for sharing! Many years ago I had a terrible problem with the rear brakes locking up on my '72 Avanti during hard braking. I tried everything to solve it. I suspected the combination valve might be at fault but could not find a replacement at the time. I removed it and replaced it with an adjustable proportioning valve which solved my problem. 

[Zedman]

 

12 hours ago, Jim S said:

Great minds think alike.  I just watched the same video a little while ago! Thank you for your advice.  I ordered the tool from Amazon and it will arrive tomorrow.

Thanks Jim-  BTW what valve do you have and is it mounted on the left hand inner Frame Rail or is it up top with your Master cyl?  If its a Kelsey-Hayes unit (like my old one) it will be OLD. I wasted a huge amount of time researching this obscure little bugger only to find it was likely to be pretty well stonkered. There are some parts for it but it not all, so it's not really worth it.  I opted for the GM PV-2 valve and it was plentiful, cheap, Brass not Ferrous and a 100% bolt-up fit so long as you use some of the old fittings. Picture attached so you can see the similarities...

Edited Sunday at 03:11 AM by Zedman

[Zedman]

Jim- I dropped the ball on the subject of Proportioning/Combo Valves, and forgot to mention what ALSO needs to be done....  If you have tried to bleed your brakes without the 'Special Tool' and the Shuttle Valve has indeed moved over- then you must RESET the valve manually (i.e. re-centre the shuttle). There should be a little 'pin' on one end of your valve, often covered by a rubber cover (just visible on the brass valve in the picture I posted).  This pin may be sticking out somewhat, indicating the valve has moved over. You will need to push this back in otherwise the 'Tool' will not impinge on the groove in the middle of the shuttle. However, because your front brakes are operating, the shuttle valve will have moved inward. To help re center it, crack a bleeder on one of your Disc Calipers and give the Brake pedal a push.  This ought to simulate a front brake "failure" and the shuttle valve ought to move the other way. One way or another, just ensure the Shuttle is centered before screwing the Bleeder Tool all the way in.  Sorry for long explanations, but as an ex high school teacher, I know that you can't afford to throw doubt.

Edited 1 hour ago by Zedman