TA22 brake pedal to the floor - intermittent

[ta22fan]

Got my TA22 just about set for rego a couple of weeks ago, and then this weekend was moving the car and the brake pedal went to the floor - no brakes at all. I've previously bled the brakes and checked for leaks - all seems fine. Pretty confident that I've not got any air in the system. No noticable softness in the pedal until this happened with no warning at all. Recall this happening once before a while ago, but seemed to come good after replacing the (very old) brake fluid. Bit of on-line research suggests the problem could be a faulty booster unit. Can anyone confirm that this is a possible/likely cause before I swap it out? I've got a couple of spares available, but it'll be a biggish job - pretty sure I will have to pull the master cyclinder assembly off before I can get the booster unit out (which means another full bleed of the system when I put it back together).

Assuming it is the booster - has anyone tried stripping one of these down? I have my trusty workshop manual that shows how to go about it, but not sure that there is much point as I may not be able to fix anything worn or damaged that I find.

Paul

[scottreichstein]

the o rings in the master cylinder could be stufffed causing the booster to fill with fluid may need to replace the master cylinder

[oldcorollas]

i've had similar when the cup on the MC piston was cracked. sometimes it would hold pressure.. sometimes it would go straight to floor.

maybe undo master from booster first, and see if there is fluid leaking?

[lexsmaz]

No way a Brake Booster will do this unless the link rod internally is broken, which i have never seen happen, plus you say its intermittant ..

Basically when you put your foot on the brake, its a direct link to the m/cyl through the booster, but roughly half way inside the booster there is a Diaphragm which is also connected to the rod which then assists you with the effort required to press on the m/cyl ( with the engine running that is ) ..

As has been said it should be the m/cyl, but check you are not going through a lot of fluid as it could be a bad leak somewhere else in the system if you are going through a lot of fluid i would be looking at other areas too, if the internal seals in the m/cyl are leaking it will lose fluid but generally not heaps ...

Do you find if you gently rest your foot on the brake pedal it goes to the floor, but if you punch it fairly hard it stays good, if thats the case it definatley is the m/cyl ..

[oldcorollas]

Do you find if you gently rest your foot on the brake pedal it goes to the floor, but if you punch it fairly hard it stays good, if thats the case it definatley is the m/cyl ..

mine did that too! also, push hard = solid, back off a little and it would sink to floor as fluid leaked past.

could also be rear drums retracting too much for some reason?
I'd suggest not driving it until you find the cause

[feral4mr2]

^^ a pitted bore in the MC will cause that to happen. i prefer to get the MC lined with a stainless steel sleeve so it cant pit again. or you could just change your fluid more often so the moisture that gets into the brake fluid cant sit there and pit the bore.

[ta22fan]

Thanks guys. I was having trouble with the idea that the booter could fail in this way - seemed logical that it would fail-safe, and just leave you with unassisted brakes - which would still work, just not as well. Checked again this morning - still nothing - basically no resistance on the pedal all the way to the floor (just the return spring) - much less than you feel when bleeding the lines. I can push it all the way down with a finger. Pumped it 4 or 5 times, and no change to the fluid level. Figure if she had a leak somewhere, I would see a drop in the level after doing that, so thinking it must be something seriously wrong with the master cyclinder. I'll swap it out this weekend. Will let you know what I find.

[oldcorollas]

spring pushing the MC piston back has busted?

if there is no resistance, either one of the brake pistons (front or rear) is retracting, and getting pushed out by the MC but not contacting enough to give pressure (rear shoe adjuster busted/backed off?),
or, the MC piston is fully pushed in and not springing back (but the fluid in front of the piston still has to go somewhere...)
or, hole from fluid reservoir to MC bore is blocked, so when piston pulls back, it can't replenish fluid used to adjust for brake wear,
umm.. couple of idea to check..

[lexsmaz]

Also stick your head under the dash & just make sure the clevis pin has not fallen out from the brake pedal to booster link rod, ie just make sure when you push down the brake pedal it is connected with the rod the goes through the firewall into the booster ..

[ta22fan]

Good point - would be feeling pretty foolish if I pulled the brake system to bits to find that something this simple was the cause.

[lexsmaz]

Again it wont happen by itself unless someone has been playing under there in the past & has not put back the circlip on the pin, but worth checking especially as you say how little the resistance is ..

[ta22fan]

Checked the pedal linkage - all good. Swapped the master cyclinder for a spare, and bled the system (went through a full bottle of DOT 3 to get the air completely out of the system) - brakes are back! Nice firm pedal, and I can hold with firm foot pressure and no sign of any pedal movement. Pulled the piston assembly out of the old MC, but no sign of any obvious problem with piston/seals or the bore - just a bit of normal wear. Certainly nothing that would explain zero pressure. Beats me. Might get it overhauled and swap it back - can't afford another failure with these single piston units.

[lexsmaz]

Good to hear thats its all back to normal, i didnt realise it was a singlular braking system, hence why the pedal went too the floor ..