Brake booster works intermittently (1 Viewer)

[Plantman]

Hi, I recently acquired my dad's 1977 FJ40, which has sat for the last 8 years. We got it running, and we've had an interesting time with the brakes. It has a brand new master cylinder and the system is very well bled. At times it feels like the brake booster works great, at other times it does nothing and all the pressure on the brakes has to come from the driver, kind of hard to get the rig stopped with 38" TSLs on it...

Anybody had a similar problem or have any idea how to fix it?

Thanks

 

[447xpro]

Brand new or rebuilt?

 

[Mace]

How's the check valve?

 

[Plantman]

As far as I know, it's the original booster. How do you check the check valve? And would a faulty check valve explain why sometimes the booster works and sometimes it doesn't? It doesn't seem to make any difference is the engine is cold or up to operating temp, if that has any relevance.

 

[Landpimp]

Could be the old master leaked into the booster and it's on it's way out, we had one that we knew had fluid it in(or at least the master was leaking into booster), sometimes it worked, sometimes it didn't. Replaced and no issues

 

[NKC]

May not be the problem but easy to check, it's a one way valve.
The valve should allow flow from the brake booster to the intake manifold to create vacuum. So disconnect the hose at the intake manifold and blow through the hose. If air passes through, replace the booster check valve.
Eliminates one variable.

 

[Plantman]

May not be the problem but easy to check, it's a one way valve.
The valve should allow flow from the brake booster to the intake manifold to create vacuum. So disconnect the hose at the intake manifold and blow through the hose. If air passes through, replace the booster check valve.
Eliminates one variable.

I'll give this a shot. What Landpimp says makes sense too, since my master cylinder was leaking from the back.

 

[thebigredrocker]

Hopefully it’s just a bad check valve.

If you think you might have fluid in the booster, you can pull it, pull the one-way check valve out of the grommet, pull the grommet and set the booster down so that the grommet hole is the lowest point. If anything drains out, the verdict is clear.

 

[Mace]

The reason I bring up the check valve is that brake boosters work off vacuum. So, if the check valve is faulty and you've been on the gas, you do not have much Vac accumulated in the booster to give the "boost"

Does it happen at a specific time?

 

[Plantman]

The reason I bring up the check valve is that brake boosters work off vacuum. So, if the check valve is faulty and you've been on the gas, you do not have much Vac accumulated in the booster to give the "boost"

Does it happen at a specific time?

I'm not sure. I've been driving my dad's 80 for the past week, but I'll be around the 40 this week and I can pay attention to that when I drive it.
Thanks everyone for the suggestions. I think I'll pull it off and see if I can drain any fluid out.

If brake fluid were in the booster, would that damage it permanently or only while it has fluid in it?

 

[SteveH]

If brake fluid were in the booster, would that damage it permanently or only while it has fluid in it?

Fluid inside the booster will not necessarily damage it. You should remove and drain - preferably overnight.

Another way to test both the check valve AND the grommet is to simply drive the truck, park it, wait 10 minutes, and then try to remove the check valve from the booster (carefully, so you don't crack any plastic parts). If you hear a 'whoosh', then the booster still had vacuum in it, and the check valve and grommet were sound/intact. If the booster won't retain vacuum for more than a couple minutes, there's a leak in the system.