brake pedal to the floor

[1lo620]

Ok I just repalced a brake line in the rear. So I went to bleed the brakes and all seem fine. I thought I had them bled fine but when i went to test drive it, the intitial push on the brake pedal and it goes to the floor...but if I pump it a few times, all is good...Did I just not bleed them all the way, possible air in the lines still. Brakes worked fine before I changed out the brake line. And yes, the resevoir did go dry,,,Any suggestions or advice....thanks

[Guest Rick-rat]

Bleed again, and don't let the reservoir go dry. sounds like you still have air in the line

[q-tip]

^^^^do this, could be air in the lines or the master cylinder. If there are bleeders on the master cylinder start there then the brake farthest from it and finish with the brake closed to it and do the master again.

[banzai510(hainz)]

bleed again per Q Tip.

On 521 s brakes are NOT self adjusting so one has to take the slop out also. If was fine before then bleed

[KA620KC]

I know this is an old thread but I'm having the same exact issue on my 620 after having to replace both rear brake lines. I've tried bleeding the brakes 4 times and I still get the pedal to the floor until I pump it 3 or 4 times. I even bought a Vacuum pump to try to suck the air out of the line. Any suggestions? and did rebleeding work for 1lo620??

[ggzilla]

When you replaced the rear line, did the fluid drain out of the master cylinder? If so, you need to bleed the master cylinder first. Otherwise you'll never get it bled properly.

[KA620KC]

When you replaced the rear line, did the fluid drain out of the master cylinder? If so, you need to bleed the master cylinder first. Otherwise you'll never get it bled properly.

 

No I don't think it leaked that much out but I bled it from the Master first then did the rears like the FSM says. I did however make a loop in the rear line because it was too long and I didn't have my flairing tool. would that cause me to have a air pocket that couldn't make it out??

[datzenmike]

Be sure your brakes are adjusted on the back.

[KA620KC]

Be sure your brakes are adjusted on the back.

 

Done, that was one of the first things I did when I installed the new shoes. I'm going to rebleed one more time after taking the loop out of the line that I made and see if that doesn't help. And if it doesn't then I'm going to replace the MC.

[datzenmike]

The loop shouldn't matter.

 

Was the line replaced on the rear axle or closer to the NLSV.

[KA620KC]

NLSV?? I'm not sure i know that is. But I did replace both rear brake lines from the Tee. I'm gonna try AGAIN to bleed it tomorrow. So far i've bled it 5 times.

[datzenmike]

Sorry.... Nissan Load Sensing Valve, it's that thing under the pass seat on the inner frame.

[wayno]

I have found that when bleeding the rear circuit of a dual master brake cylinder, that the front circuit interferes with bleeding the rear circuit, fact is on the last one I did, I had to open the front circuit to bleed the rear one, what I did was just barely open one of the front bleeders on the driverside, stick a small clear hose over the bleeder(aquarium air hose) and put it into a partially filled cup of brake fluid, then I bled the rear circuit making sure that neither master brake cylinder reservoir gets low, and after the rear circuit is done, I then I closed the front bleeder.

This is the very reason why you always do the rear circuit first.

[KA620KC]

I have found that when bleeding the rear circuit of a dual master brake cylinder, that the front circuit interferes with bleeding the rear circuit, fact is on the last one I did, I had to open the front circuit to bleed the rear one, what I did was just barely open one of the front bleeders on the driverside, stick a small clear hose over the bleeder(aquarium air hose) and put it into a partially filled cup of brake fluid, then I bled the rear circuit making sure that neither master brake cylinder reservoir gets low, and after the rear circuit is done, I then I closed the front bleeder.

This is the very reason why you always do the rear circuit first.

 

Gotcha..... I just had a mechanic friend of mines tell me to do the exact same thing. I'll try that tomorrow after work

[KA620KC]

Since I might be replacing the MC is there an upgraded MC I could rob from a different car to use?

[ggzilla]

No, the 1977 620 has an optimal Master Cylinder.

[datzenmike]

What does upgraded mean? Your 3/4" MC is carefully designed to match the other brake components. A larger 13/16" master from a '78 620 will move more fluid but will increase peddle effort to do so. The peddle will feel stiffer and more effort is required to stop.

 

I replaced the stock 3/4" master on my 710 with a13/16" from a 200sx. The old one was bad and my parts car one was the same. They all leaked or wouldn't push fluid. Peddle is rock hard now and while it stops just fine it does take a little more effort.

[KA620KC]

What does upgraded mean? Your 3/4" MC is carefully designed to match the other brake components. A larger 13/16" master from a '78 620 will move more fluid but will increase peddle effort to do so. The peddle will feel stiffer and more effort is required to stop.

 

I replaced the stock 3/4" master on my 710 with a13/16" from a 200sx. The old one was bad and my parts car one was the same. They all leaked or wouldn't push fluid. Peddle is rock hard now and while it stops just fine it does take a little more effort.

 

I was just curious to see if there were a more optimal MC to use. But if the factory one is sufficient then I will stick with it. Thank you for the input though!

[Tonka]

Datsun 1000 B20 ute, 4 wheel drums and a single circuit master cylinder, lost all brake fluid yesterday after the flexible hose to the diff split and the pedal hit the floor. Went to a friends place real close by and he helped fix it. Then when we went to bleed the rear brakes he looked at the rear bleeder nipples and said "why is the bleeder nipple located lower than the brake line, how does that allow the air to escape ?" Good question. We looked at swapping the rear wheel cylinders but that wouldn't change the bleed screw positions, looked at rotating the cylinder 180 degrees but then the handbrake mechanism wouldn't line up and looked at swapping the upper hydraulic line with the lower nipple but they are clearly different threads. Ended up parking backwards on a steep slope and jacking the front up about a meter to get the line and the nipple positions just level to get the air out as we pressure bled it. The front wheel cylinders have the bleeder nipple at the top so no issue with conventional bleeding techniques.

 

So the question now is, is this all normal ? Or maybe have the rear wheel cylinders been replaced previously with another type that fit perfectly but are designed for another vehicle to mount at some other position ? Anyone got any clues as to what's going on ? Or seeen/heard of a similar issue ?

[ggzilla]

Yes thats normal. Conventional bleeding technique works on it, the volume is very small.

 

I would change to tandem master, so a leak does not cause all four brakes to fail. I think B10 Deluxe came with tandem brakes.

[Tonka]

Thanks, if that's the correct original engineering design then I'm just amazed and now a bit more curious. Inside the wheel cylinder is there any fancy porting that somehow directs the air from the airspace at the top of the cylinder down to the bleed point ? I can't get my head around why any engineer would design something that would defy the underlying principal of liquid to the bottom and air to the top. To save me pulling my wheel cylinders out and looking, can you remember if the ports for the hydraulic line and the nipple are just plain holes drilled straight into the main body cavity of the cylinder straight in line with where the respective line and nipple screw in ?

 

And yes a dual circuit master cyl is now the plan.

[ggzilla]

You are over thinking it. Volume and pressure forces both fluid and air out

[Tonka]

The brakes work again, but have always felt spongy. I still imagine that any air in the sytem will always go to the top and liquid to the bottom, I don't understand why an increase in pressure would make the air fall down into the liquid. As the bleed nipple appears to not be at the very top I still can't get how the air it's meant to remove gets down through the liquid to where it's positioned on these rear wheel cylinders. It will have to remain as one of life's mysteries for me for now. Thanks for your help.

[ggzilla]

Adjust all 4 brakes to spec, and the sponginess will go away.

[datzenmike]

Be sure that the rear drum shoes are adjusted out as far as possible without dragging. The further the shoes have to move before contacting the drum, the further your brake peddle has to move.