mikliska

I have a '79 that has suddenly developed a serious brake issue!
I bled the brakes after replacing the master cylinder and got a nice firm pedal and told my assistant "thank you" and sent them on their way. I went for a test drive but didn't make it out of the drive-way. When I touched the pedal the first time after revving and releasing the clutch to get her going the pedal went straight to floor. once I got it stopped I set the parking brake and pumped the brakes... nice firm pedal, rev the engine and let it return to idle... no pedal. Pump again, firm pedal. Release parking brake and after same procedure... no pedal. I bled the brakes again... same problem. removed rear drums and check cylinders and found no leaks. Same with the front... afterall I get a firm pedal until I rev it. If I remove the vacuum line from the booster I can hear the air rush in. No apparent vacuum leaks on the other side of the check valve (not many line left).

Idle does change when applying brakes.

Any ideas? Any help would be deeply appreciated!

installer67

iTrader: (18)

As far as I know, the booster is the only part of the braking system that could affect or be affected by idle, think you may have a problem with it. Check your FSM for procedure to test it.

mikliska

Thanks! I thought so too. Anyone else agree?

80's old school

iTrader: (7)

When you changed the booster did you remove the master and discoonect the brake pipes?

You probably have air in your system still and a bunch of it.

Your booster is there to ONLY make the pedal easy to push. Even if your booster failed completely, the brakes should still work.

Kentetsu

How sure are you that you are bleeding the brakes in the correct manner?



.

t_g_farrell

iTrader: (1)

You either have a ton of air in the brakes or you booster is not
holding vacuum. So make sure to bleed the MC (yes SAs have a
bleed on the MC) and then do the RR,LR,RF, and LF wheels in that order.

One test is to put some pressure on the brake pedal before
starting and when it starts the pedal should drop a bit but then
stay firm. If it doesn't its a bad booster.

80's old school

iTrader: (7)

I would not think a bad booster would make the pedal go to the floor. It is only there to "power assist"

mikliska

Thanks for all the replies buuuut:

1) I didn't change the booster

2) I did bleed the master first and even bled the hard lines

3) I did bleed starting at the rear and ending LF

4) I didn't bleed the LR because there is no bleeder on drums, though I guess I coul try to bleed the hard line.

5) I do have a firm pedal until I drive away

I have bled many brakes and never had any problems unless there was another problem in the system. I even successfully bled the brakes for a certain rx7 "guru" who had thrown his wrench down in frustration. I only mention this to establish my confidence in the bleeding with the exception of the LR (but, once again, firm pedal until I move).

If I'm overlooking something please let me know. I'm afraid to change the booster without being sure as tose things aren't very cheap.

Thanks for checking back.

t_g_farrell

iTrader: (1)

Heres what I went thru on my booster. A new one fixed me up BTW.

https://www.rx7club.com/1st-generation-specific-1979-1985-18/bad-brake-booster-679357/

BeenJaminJames

If the pedal goes to the floor and the hydraulic lines don't pressurize, then the booster probably is'nt the problem. Who manufactured your master cylinder? I have had many similar problems with aftermarket "OEM" master cylinders. Usually the problems are caused by the pressure equalization port not lining up with the piston where it's supposed to. This has alot to do with the booster pushrod adjustment, so I'd re-check that first. Most manufacturer replacement parts (AC Delco, Motorcraft, etc) are not produced to the same specifications that the original equipment was. The cheap stuff you get at parts stores is even worse. Who manufactured your master cylinder? How much pedal free play do you have? How much is the drop when you start the engine? With the engine off, push the pedal down and keep it there. After a few seconds, do you still have full pressure at each wheel? (park on a hill, kill the motor, stand on the pedal, release the handbrake, wait a few seconds, see if the car rolls) If everything is adjusted right and you are losing pressure, (and you dont have a leaking line somewhere) then your master cylinder is probably a dud.