Pedal sinks - M/C? DIY Rebuild?

[dogbreath_48]

Had a search but couldn't really come up with much.

My brake pedal slowly sinks to the floor under pressure, then hardens up with a pump. I assume it's the master cylinder leaking but I can't trace a leak. I would have expected the rubber boot to be full of fluid. Wheel cylinders appear fine, car doesn't pull etc. Is there anything else it could be? (before i go to the effort of stripping bits down) I'm uincertain as the previous owner said the M/C had been rebuilt. The problem has only become noticable in the last few days

Assuming it is the M/C, is a rebuild at home do-able? Can i buy a kit? Is it likely the bore will be stuffed and require machining/sleeving? Trying to budget

Car is an '87 HJ75

-Stu

[+dj_hansen+]

you can buy rebuild kits for M/C, and they appear pretty easy to rebuild DIY (reading through my gregories manual), depending on how out of round the bore is.... then it comes down to sleeving or buying new... maybe pull it apart (if you undoe the circlip at the end it should all come out) and take it to bursons/repco/brake shop and see what they think....

[-Scott-]

Had a search but couldn't really come up with much.

My brake pedal slowly sinks to the floor under pressure, then hardens up with a pump. I assume it's the master cylinder leaking but I can't trace a leak. I would have expected the rubber boot to be full of fluid. Wheel cylinders appear fine, car doesn't pull etc. Is there anything else it could be? (before i go to the effort of stripping bits down) I'm uincertain as the previous owner said the M/C had been rebuilt. The problem has only become noticable in the last few days

Assuming it is the M/C, is a rebuild at home do-able? Can i buy a kit? Is it likely the bore will be stuffed and require machining/sleeving? Trying to budget

Car is an '87 HJ75

-Stu

Unless he/she lied, another seal kit won't help. I've always (two or three times...) had stainless sleeves inserted, and never had to touch a sleeved cylinder.

[pongo]

i bought my MK patrol m/c for about $80 not worth the mucking round i reckon, but its up to you.

cheers

[gorilla]

check wheel bearings first! was the source of my problems. because the bearings were loose the disc mould puch the pad back inside the caliper, the first pump is just to push the pad to the disc. went from perfect brakes one day to almost no brakes after 4wding --> because the frontwheel bearings copt a beating

[dogbreath_48]

check wheel bearings first! was the source of my problems. because the bearings were loose the disc mould puch the pad back inside the caliper, the first pump is just to push the pad to the disc. went from perfect brakes one day to almost no brakes after 4wding --> because the frontwheel bearings copt a beating

After the 2nd or 3rd pumps, the pedal should stay hard should it not?
I'll check my bearings anyway just in case.

[pongo]

and the rear drums if you have em, that was the culprit of the last one i think.

Cheers

[RockyF75]

It's Not the slave/ wheel cylnyders?? easy to check if they leakin

[pongo]

Nah , the lwb i bought had new pads in the rear, but werent adjusted. Pedal too 4 pumps to build up pressure.

Fixed them after i replaced the m/c then had no dramas. Maybe worth a try.

[dogbreath_48]

I'll pull the wheels off and check out the bearings/pads/shoes/pistons etc.

This couldn't be caused by a vacuum/booster problem could it?

[-Scott-]

I'll pull the wheels off and check out the bearings/pads/shoes/pistons etc.

This couldn't be caused by a vacuum/booster problem could it?

No.

Everything points to the master cylinder.

Over time, crud has collected in your master cylinder, and settled to the bottom. Corrosion started (probably something to do with moisture in the fluid too) and has now reached the stage where the seals are not effective.

As you push the pedal first time around all the caliper pistons/wheel cylinders fill and apply pads/shoes - by which stage the piston in the master cylinder has hit the corroded spot, and fluid starts to leak past the edge of the cup, but stays inside the master cylinder. As fluid leaks past the seal the piston slowly travels along the cylinder, and your pedal slowly sinks to the floor.

When you release the pedal, the cup travels back along the bore, and any fluid which had leaked past on the "pressure" stroke finds its way back past the seal (cup seals only seal in one direction) - so no visible leak from your master cylinder.

When you push again, less fluid is displaced into the brake lines (because pads/shoes are already out), so the piston doesn't travel as far, and doesn't hit the corroded spot - so the seal holds the pressure, and you have a firm pedal.

The problem is in the master cylinder - this is why the previous owner rebuilt it, but didn't do the job properly. So now it's your problem.

If the leak was anywhere else, you would be losing brake fluid from your reservoir - are you?

Scott

[dogbreath_48]

I'll pull the wheels off and check out the bearings/pads/shoes/pistons etc.

This couldn't be caused by a vacuum/booster problem could it?

No.

Everything points to the master cylinder.

Over time, crud has collected in your master cylinder, and settled to the bottom. Corrosion started (probably something to do with moisture in the fluid too) and has now reached the stage where the seals are not effective.

As you push the pedal first time around all the caliper pistons/wheel cylinders fill and apply pads/shoes - by which stage the piston in the master cylinder has hit the corroded spot, and fluid starts to leak past the edge of the cup, but stays inside the master cylinder. As fluid leaks past the seal the piston slowly travels along the cylinder, and your pedal slowly sinks to the floor.

When you release the pedal, the cup travels back along the bore, and any fluid which had leaked past on the "pressure" stroke finds its way back past the seal (cup seals only seal in one direction) - so no visible leak from your master cylinder.

When you push again, less fluid is displaced into the brake lines (because pads/shoes are already out), so the piston doesn't travel as far, and doesn't hit the corroded spot - so the seal holds the pressure, and you have a firm pedal.

The problem is in the master cylinder - this is why the previous owner rebuilt it, but didn't do the job properly. So now it's your problem.

If the leak was anywhere else, you would be losing brake fluid from your reservoir - are you?

Scott

This is what i assumed.
I am losing a tiny bit of fluid, but that could just be the fluid that isn't recovered as the piston recedes, correct?

Danm, i really can't be fucked rebuilding my M/C

-Stu

[Struth]

Could be as simple as a vacuum leak.
check the vacuum line.

Struth

[-Scott-]

Could be as simple as a vacuum leak.
check the vacuum line.

Struth

I'm sorry, I'm a little slow tonight.

When you lose vacuum to a booster the pedal pressure increases, and typically pedal travel reduces.

Could you explain how a vacuum leak will cause the pedal to sink to the floor?

Perhaps it's water in the petrol? Or sand in the gears? No, wait, check your wiper shims!

I'm sorry. I'll go start drinking now.

[RAY185]

Agree with Scott, replace the MC, better off buying new they are not that exy and we're talking brakes...safety....not to be skimped on.
You wont see the leak till you pull it off and find brake fluid inside the booster.

[Struth]

As I have said before "no brakes = no fun"

If it can't be diagnosed on the forum, see the experts before driving it too far at all.

Cheers Struth.

[Nev62]

I'll pull the wheels off and check out the bearings/pads/shoes/pistons etc.

This couldn't be caused by a vacuum/booster problem could it?

No.

Everything points to the master cylinder.

Over time, crud has collected in your master cylinder, and settled to the bottom. Corrosion started (probably something to do with moisture in the fluid too) and has now reached the stage where the seals are not effective.

As you push the pedal first time around all the caliper pistons/wheel cylinders fill and apply pads/shoes - by which stage the piston in the master cylinder has hit the corroded spot, and fluid starts to leak past the edge of the cup, but stays inside the master cylinder. As fluid leaks past the seal the piston slowly travels along the cylinder, and your pedal slowly sinks to the floor.

When you release the pedal, the cup travels back along the bore, and any fluid which had leaked past on the "pressure" stroke finds its way back past the seal (cup seals only seal in one direction) - so no visible leak from your master cylinder.

When you push again, less fluid is displaced into the brake lines (because pads/shoes are already out), so the piston doesn't travel as far, and doesn't hit the corroded spot - so the seal holds the pressure, and you have a firm pedal.

The problem is in the master cylinder - this is why the previous owner rebuilt it, but didn't do the job properly. So now it's your problem.

If the leak was anywhere else, you would be losing brake fluid from your reservoir - are you?

Scott

Outstanding Tech advice. I have the same sort of problem and could not workout where it is leaking as there is no fluid loss and no air in the lines. What makes you post so good is the explination of what is/could be happening and why there would be no fluid leak.

[lethal weapon]

I replaced my M/C on Friday, new one- $180, rebuild kit- $143, didn't think it was worth the stuffing around... Good as gold now, goes hard after one pump... Oh and the M/C works well too!!!