Help: Sinking Brakes

[chosen1]

Well Dave that means the booster is good. The more I think about it the more I feel like I should change the check valve, gonna try to bleed the MC itself and see if it works..

[Dave]

Exactly what I was hinting to you. You need to establish that pressure built up at 4 corners is the same being expelled. Check valve meaning Proportioning valve?

[racedriverpro]

Probably the check valve in the mc vacuum line.

[Dave]

Check valve won't cause that. If check valve sticks it would cause pedal to be stiff since booster not getting vacuum.
Check valve ensures that oil doesn't go to the mc from blow by of the engine.

[racedriverpro]

Check valve won't cause that. If check valve sticks it would cause pedal to be stiff since booster not getting vacuum.
Check valve ensures that oil doesn't go to the mc from blow by of the engine.

I knw...just trying to clarify what op was referring to

[chosen1]

Yeah, I looked at the combination valve, but it's an old model and I am not seeing the orifice to open when bleeding the brake system. I also noticed a seal that came with the MC (for under the cover) was not installed.

So I installed it and will check again tomorrow.. I am hoping that thats where the pressure was escaping.

[chosen1]

Check the combination valve.. Which is really more of a distribution block (2 inlets 4 outlets). Cracked all the lines to reconfirm it was distributing (which I was not doubting since I was getting fluid at all four corners when bleeding)

Bled the MC at the two outlets (no air released) just as a double check.

All research would indicate that I got a faulty MC, even though I purchased new. Will purchase another at another retailer on Wednesday.

So far I am sure the booster is good. I am sure the brakes themselves are in good shape with no leaks. The distribution block is functional. I have no leaking lines.

[dude2014]

Well well...after replacing the cylinders, shoes, even bleed valves in the rear brakes the pedal still sinks to the floor.

Bled the rear brakes and was disappointed when the pedal still reacted they way it did. So now it's onto the front brakes... Since I know the rears are good.

Pulled down the front driver side only to realize that the caliper was stuck. When I tried to push it back it did not move until I opened the bleed screw. The slide pins were dry also. I am thinking that I will replace the rubber brake line as well as it looked like the original piece and may be broken down on the inside... Will post more when I button it back up later this week.

Hope this will help. The front calipers has a piston which will appear to be stucked. The problem is crap gathers behind the piston and when you attempt to push it in, the crap clogs the hole. Remember the hole which allows fluid to flow behind the piston to activate the Disk pads is as small as a fine point pen. Remember brakes fluid performs because it is not supposed to compress when the brake pedal is stepped on and the fluid because it cannot flow backwards will make the piston appear to be stucked.

It is necessary to press the piston out, clean behind the piston and re- install. You may need to get a caliper kit with the rubbers to replace if they are old. It will also give you a good opportunity to inspect the piston and cylinder. When cleaned the fluid will flow properly and the system can be properly bled leading to "good braking/stopping power""

What really causes the soft pedal is the air in the line which cannot pass through the hole to be 'bled' from the system. The front calipers alone can account for the phenomenon of the soft pedal. Service both Caliper PISTONS by pressing them out (ie pumping up the pedal until they fall out).

Had the same problem on my Primera and was corrected as such. Do not sand the pistons as that is the jackass advise you get. If it is rusted or pitted, then get good used or new ones.

Give me a hail on the results ............

[chosen1]

Thanks for the advice. I did a complete service on all brakes. Changed pads, bleed valves, cylinders in the rear. Same with the front, pads, pistons, slide pins, boot seals, new brake lines, bleed valves

At this point though I really think the MC is faulty, but I could be wrong.. I am open minded to any solution

[Dave]

Bench bleed the mc and see the pressure that you get and then you will know if it is holding pressure

[chosen1]

Thanks Dave...i did not think of bench bleeding

will try it

thanks

[Dave]

You spoke about new mc. New from where? I bought a new mc and still not building pressure and have found that the rubber used in these are not that great so we built the new mc with with rubbers also. Problem solved.

[chosen1]

New from The Motorist.. It's not OEM though. Will see if I get a kit from Federal or Faiz as a backup plan

[chosen1]

Success

I "bench bled" the MC while still in the car and that i believed solved the problem. I got two ends of brake line with the flare nuts from my friendly neighborhood FU parts place, zip tied two pieces of clear hose onto the other ends, screwed them onto the MC with the clear hose ends into the reservoir below the brake fluid line, pumped it with the brake pedal. Air bubbles like crazy!!

After about 10mins i got all the air out, reconnected the brake lines and bled all four corners again. Took the car for a spin on the street and the car had good brakes. I was thinking the pedal is a bit low and i think maybe i will hafta look at the slide pins. But i am getting good brakes and i am happy. Admittedly i did not go above 15km/h on the test, but i tested it reversing up an incline and on the flat, and i am happy with it.

thanks for all of the suggestions and advice.