Brake problem

[69camaro]

Having some brake problems. Set up is standard disk front and drum rear. I completely rebuilt the front end last year but havn't touched the rear.
My problem seems to be when lightly braking or sitting at the lights the pedal initially feels fine but then becomes soft and sinks to the floor. If I take another bite at it it feels fine. Under hard braking it doesn't seem to be as much of an issue.
Ok, my first thought was air in the system. Tried bleeding all four corners but wasn't able to bleed LH rear (no fluid comes out the bleeder even with the nipple entirely out though the break shoes do move). Noticed the rear cylinders could use replacing so will do that and try and sort the blockage to LH rear then try bleeding again.
Next thought was possibly Master cylinder. It's an original master so who knows if or when it was last rebuilt.
If anyone has any info or advice or if there's something obvious i'm missing I'd love to hear. Thanks.

[EAT-30T]

So does it sink slowly when your say sitting at the lights??? If your not losing fluid and its doing that I'd say master.... If your pedal sinks a bit upon initial application but is fine when you go for it straight after you may need to adjust the rear shoes....

Of course all the above is assuming there's no leaks... to check the rear cylinders make sure you pull the rubber boots back!

[cluxford]

Sounds like vacuum loss in the booster

[69camaro]

So does it sink slowly when your say sitting at the lights??? If your not losing fluid and its doing that I'd say master.... If your pedal sinks a bit upon initial application but is fine when you go for it straight after you may need to adjust the rear shoes....

Of course all the above is assuming there's no leaks... to check the rear cylinders make sure you pull the rubber boots back!

Yeah it sinks slowly and has no leaks.

[69camaro]

Sounds like vacuum loss in the booster

Hey Chris, any thoughts on how to trouble shoot this?

[cluxford]

Not really, I've had in the past a need to check the hoses etc...but I have never had to pull apart of replace a booster itself, juts sounds like on low revs it isn't getting enough PSI...I'd start with the hoses but if they were the problem it would be an issue at all speeds

[69camaro]

Not really, I've had in the past a need to check the hoses etc...but I have never had to pull apart of replace a booster itself, juts sounds like on low revs it isn't getting enough PSI...I'd start with the hoses but if they were the problem it would be an issue at all speeds
Yeah I did check hoses and all seem to be Ok.

[RSZ28]

So does it sink slowly when your say sitting at the lights??? If your not losing fluid and its doing that I'd say master.... If your pedal sinks a bit upon initial application but is fine when you go for it straight after you may need to adjust the rear shoes....

Of course all the above is assuming there's no leaks... to check the rear cylinders make sure you pull the rubber boots back!


Agreed on Master Cylinder. Master cylinder internal seal has gone no doubt. Fluid from front circuit leaks out under pressure. This is scary when it happens at the lights and you (with an auto) then start to accelerate away while pushing the brake pedal even harder....:eek: :eek: ...and even more scary when you are cruising along and need the brakes quickly. :eek: :eek: :eek:

Either a seal kit or a new MC. Have been down the new MC route... there is some crap out there.

[69camaro]

Thanks for the responses guys, much appreciated. Will let you know how I go.

[EAT-30T]

Sounds like vacuum loss in the booster

Vacuum loss will generally give a "hard" pedal as there is no vacuum assistance.... not a soft pedal.

kind of like if you pump the pedal with the motor off (no vacuum) the brake pedal goes hard......

[blackoutsteve]

That's what I'm thinking too.

If your pedal is becoming soft while stationary, it sounds as if your engine is just building vacuum and helping too much!
Do you have a vacuum gauge?

I'll bet if you remove the vacuum source (switch off the engine) your pedal will continue to remain hard. If not, then fluid is escaping.
If the master is leaking, it can leak into the cavity between the master and the booster.
Are the fluid levels in your reservior changing?

To see if you are losing brake pressure, try stopping on a fairly steep road and see if you begin to roll when your pedal reaches the floor and feels light. Have a friend get out and try to push!

Have you got fluid through the LH rear yet?