I cannot bleed all the air out of my brake system. Its a long story but I have replaced everything in the brake system except the hard lines and rear rubber hose(on axle). Initially the booster was bad and I replaced it. I have had 3 different master cylinders on it, replaced both rear wheel cylinders, shoes, etc. The vacuum pump is good, no leaks in system, the old ABS cylinder is bypassed, brake shoes are adjusted right, front calipers and hoses are fairly new. I have spent hrs bleeding system and get it almost air free but not quite. After bleeding, and before starting motor, the pedal will get hard. As soon as I start motor, pedal will go to floor. I understand a little loss of pedal after start is normal. I am at wits end; any new ideas?, thanks, Carl.

 

Did you adjust the brake rod

 

No, I did not. I thought the rod comes pretty well preset w/new booster.

 

In a perfect world my friend. I would try adjusting that and see what you come up with.

 

Welcome to FTE.

 

When I purchased a new vacuum booster for my old '92 F350 w/5.8L engine it had instructions for how to set the pushrod length. I ended up lengthening it about .045" to get it correct.

 

The replacement brake booster I installed has non adjustable rods on both sides. I bled the brake system again and have pretty good brakes but pedal still slowly goes nearly to floor.

 

Did you bleed the master cylinder(s) on the bench first??

 

It sounds like the replacement booster us bad. Check the rear soft lines are the wet or damp looking if so replace them

 

Here's a quick check of the booster I posted for someone else yesterday:



Is that new brake booster functioning properly?

With the engine "OFF", depress and release the brake pedal 4-5 times.
Depress the brake pedal firmly and start the engine.
If the pedal goes down slightly the brake booster is operating properly.
If the pedal does not go down slightly the brake booster is not assisting in brake operation. Verify vacuum supply to the booster.

Maintain pressure on brake pedal while turning "OFF" engine. The pedal should remain in the same position. If the pedal slowly slowly rises, the booster or vacuum check valve is leaking.
Verify the brake booster vacuum line is intact and installed properly. Verify that the vacuum check valve allows flow only towards the intake manifold, and not in reverse towards the booster.
Verify that engine vacuum is at least 17 in hg.

 

The new replacement booster is good, the front 2 brakes hoses are new, didn't replace rear one as I don't believe its bad as I am able to get strong flow to both rear wheels while bleeding. There are no leaks anywhere. I've been told repeatedly that there must be an air leak into system but with a new master cylinder putting pressure on system, wouldn't there be a fluid leak? I've been able to bleed most of air out of system, but not all. The air seen is mostly at left rear. The brake pedal travel is ~1-1/2"(normal), and brake pedal is hard until I start motor, then it slowly bleeds off. I've been told by several mechanics that that is normal( on Ford trucks), but I can't believe it. It is very unsettling, to say the least. Yes, I bench bled the master cylinders before installing them. I've tried 3 total. Truck has a motor driven vacuum pump and it provides strong vacuum.

 

as long as you have air in the system, you will not have a firm pedal. it takes time to get it all out.

you need to bleed the brakes by passenger rear first, then driver rear, then passenger front, and lastly driver front.

 

Ok -- left side rear...........have you inspected the proportioning valve inside of LH frame rail behind the No. 1 crossmember (for leaks)? And separately the pressure differential valve??

 

There are no fluid leaks anywhere. The RAB antilock brake valve has been bypassed(inside frame rail front left). I have spent hours and hours bleeding the brakes(whenever I could get a helper); how much time should it take? Of course, I start with right rear, left rear, right front, left front.

 

ok, lets go back to basics. did you bench bleed the master cylinder before putting it on?
if not, you will never get all the air out.
first thing i do when doing a full brake service is open all 4 bleeders, and remove the lines from the master. then using compressed air i blow all the old fluid out of the brake lines and calipers/wheel cylinders.
after bench bleeding the master, i hook everything up, fill the master with fluid, then open up the passenger rear bleeder with a hose going to a jar and walk away.
then i will check it every half hour until i get fluid out of the hose.
this will give me a base point to start pressure bleeding from.