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HI, looking for some information regarding bleeding the brakes on a 1986 firebird with disc up front and drum rear. Car has been sitting for a few years so I changed the master cylinder and done an all round bleed. Minor weep from one wheel cylinder- 2 new ones on order. ( not enough to cause no pedal) Since then ive got no pedal at all, and the brake warning light is on. I bench bleed the master cylinder and its fitted with a bleed nipple, after install i gave it another bleed to ensure that it had no air in it. Ive checked the brake light switch on the proportioning valve and can confirm its on and the valve has moved. Ive got the factory workshop manual and i have rechecked the bleed process. I did a two persons bleed, and put through 2 L of fluid to clean all the old fluid out. It does say if you do a pressure bleed that you need to use a special tool for depressing the proportioning valve to allow brake fluid to the rear brakes. Im starting to think that during the bleed process the valve has moved back. I assume when i re bleed the brakes with the valve on the proportioning valve depressed using the special tool this should fix the problem??? Has anyone else come across this or had similar issues. And how have they got around it. In Australia here, and no one stocks the special tool for this job. Pictures of special tool required according to manual.
I've literally never seen that tool and I've bleed my brakes from completely empty before without an issue. Do you have a picture of where the tool is used? Looking at it you could easily make it with some scrap metal and a bolt. Are you doing a pressure bleed?
One thing you may want to do is bench bleed again but with the prop valve hooked up to the master cylinder. The reason you have to bench bleed these cars is due to the angle the master is in the car you can't get all the air pockets out of the master. By doing a bench bleed with the prop valve and the master out of the car you can get all the air pockets out, and see how well each port is flowing on the prop valve.
Just a word of caution a while back alot of people were changing the springs out on their prop valves to "help" their brakes but usually didn't quite work out as well as they had hoped.
Last edited by Aviator857; 02-07-2019 at 03:26 PM.
Cool thanks for that. My next step was to remove both units and bench bleed again. Hopefully with no pressure on the unit, the valve will re centre as well. The tool clips on the firewall side of the combination valve and depresses it allowing fluid to flow to the rear brakes when bleeding. Got this info straight out of the manual.
Easy thing to check first is to make sure your rear shoes are adjusted out against the drum. If not adjusted out against the drum, it takes a lot of fluid to get the shoes back up against the drum and you will witness a low pedal.
If you are getting a good stream out of the bleeders, front and rear, I would not suspect the low pressure switch needs to be reset. If it does need to be reset, jack up one side of the car and secure with jacks, open up both front and rear bleeders on the side you you jacked up, press and hold pedal down the pedal, then close bleeders. Do this multiple times until you here it click back to center. You’ll need two people for this, but it should reset your prop valve.
Make sure your master cylinder is holding pressure by bleeding using plugs in the Master cylinders outlets. Mount master cylinder in vice. Plug the outlets of the master cylinder with the proper size plugs. With a blunt rod, cycle the master cylinder piston while it’s secured in the vice. After a few strokes of the master cylinder piston, the piston will become rock hard with little to NO movement. This means there is no air in the master cylinder. Hold the piston down for 60 seconds. If the piston sinks down the bore, the master cylinder is not holding pressure and should be replaced. If the piston holds its position, high and hard, bolt it back on he car and bleed the rest of he system. Let me know if you have additional questions.
02-07-2019, 03:42 AM
