Mushy brake pedal after sitting for months 77 620

[abbylind]

My 77 620 has been sitting for a few months. Hooked up the battery and after pumping some fuel in the carb it fired right up.

Pushed on the brake pedal and it felt squishy. The front reservoir was low on brake fluid so I topped it off....still squishy. No visible leaks on the wheels (wheel cylinders and brake hoses were replaced 2 years ago) and I dont see anything leaking around the master cylinder...could it be an internal leak on the master? 

Inquiring minds are asking...

Thanks

[banzai510(hainz)]

I had a internal leak in the master on my toyota pick up.  I have another toyota pick up to compare to.

 

but it was a shotgun guess I did. as I never had this before so more or less only half by brakes was actually working. The previous owner drove it for years like this then I got it saying this dont feel right. But it never leaked fluid. It was just bypassing the seal internally

 

i would blled the system and tighen the drums up if all 4s as you have a 77. I dont know when they put disct up front

Edited June 9, 2023 by banzai510(hainz)

[420n620]

 Did you look inside where the brake cylinder is to see if any brake fluid seeping. If not.

I'd re-bleed the brakes but first I'd adjust them.

 

[abbylind]

Ya I think that was my next step ....pull the brake drums and recheck the cylinders. IIRC the front reservoir would be the rear brakes so Ill start there...thanks. I never thought the MC might leak betwix the front and rear systems...

[datzenmike]

 

 

Front and back systems are drum. If not adjusted up snug even a small amount out eats up pedal travel to just get the shoes into contact with the drum.

 

Before adjusting the rears, bleed them. It's easier to bleed if there is lots of wheel cylinder movement. This gets rid of the old fluid and any air that might be in the rear system. Be sure to loosen the parking brake adjustment slightly. Once new fluid is flowing and any air out set the adjustments and don't forget the parking brake.

 

 If internal leak it has to go somewhere. If front into back or back into front one of the reservoirs will over fill and the other go dry. Not likely. If the seal were bad and fluid got behind it, I should imagine on the return stroke it would get pushed back into the reservoir. Don't know but that's how I would have designed it.

 

Now check the front adjustments. The front brake piston has to move first as it's in the rear of the master. Between it and the rear piston in the front is a spring. The front moves first and eventually it compresses the spring and begins moving the rear piston. If the front drums are also not adjusted up snug there will be a lot of soft pedal so make sure they are adjusted also.

[Stoffregen Motorsports]

Drum brake wheel cylinders are notorious for sucking air when sitting for a long time. The tiny cup seals inside the wheel cylinder need residual pressure to keep them energized. I don't know when (or if) Datsun started using residual valves in the drum brake master cylinders. I do know my 320 had this problem until I installed an in-line residual valve.

[datzenmike]

I checked and the 620 does have residual valves, so did the 510, 521/520 and Roadster but I don't see one on the patrol. Can't go back any further.

[Stoffregen Motorsports]

How did you check? Is there a part number for a residual valve? They are part of the master cylinder assembly, yes?

[datzenmike]

They are on the bottom of the master one at each outlet line. The '74-'77 620 rear residual valve is 46050-B5022

 

 

I put a 15/16" master in my 710 from a zx. Had to remove the disc brake residual valve and swap the 710 drum residual into it. I could see no differences so do not mix them up.

[H3nleY]

I'd definitely bleed first to rule that out

[abbylind]

And bleed I shall! Thanks guys.....

[abbylind]

Checked the MC. All reservoirs are empty. It appears brake fluid leaked out the back of the MC. I can see a drip mark where the MC bolts to the booster.

I would like to do the disc brake conversion in the future. IIRC the larger MC is needed for the conversion? And the MC out of the 240SX would work?....or 240Z?

Any tips about installing the new MC would be appreciated....I'm a little rusty in that area...

Thanks

 

[banzai510(hainz)]

most new master cylinders(china) the shaft is too long sometimes and needs to be cut as the pedal arm will engage the master when you install to the fire wall.  so key is to cut it correctly.

i  sually put the nut on first run it all the way IN. then cut the threaded shaft close to the old one then run the nut back out to cleans the threads

[datzenmike]

16 hours ago, abbylind said:

Checked the MC. All reservoirs are empty. It appears brake fluid leaked out the back of the MC. I can see a drip mark where the MC bolts to the booster.

I would like to do the disc brake conversion in the future. IIRC the larger MC is needed for the conversion? And the MC out of the 240SX would work?....or 240Z?

Any tips about installing the new MC would be appreciated....I'm a little rusty in that area...

Thanks

 

 

The 240zx master cylinder has a horizontal bolt up pattern and won't fit the 620.

 

Your stock master will work with added front disc brakes but because the calipers pistons are so large it will take more fluid volume to move them. The master pushed by the pedal will have to travel farther and this added pedal travel will feel mushy. They will still work though.

 

Your '77 will have a 3/4" master. The '78- were 13/16" because they had disc brakes but depending on the caliper size used on the conversion you may want to go even larger like a 7/8". Keep in mind that you can't get something for nothing.... a larger master requires more effort to use. A brake booster will mitigate most of the increased pedal effort. The 620 has a brake booster so even a 15/16" is an option.

[abbylind]

Thanks guys!

[Stoffregen Motorsports]

2 hours ago, datzenmike said:

 

The 240zx

240sx

 

Unless you want to play pedal stiffness games, I'd go with the late 620 disc brake master. It is sized appropriately for a disc conversion using stock components.

[datzenmike]

The 620 calipers are not that large and a 13/16" will suffice. The late 720 where huge with vented rotors and for them Nissan went to a 15/16" master for that very reason.

 

That said I kept my stock '78 master when I swapped a Toyota 4x4 front axle in and they are dual piston calipers. The brake pedal was great.

[abbylind]

Bueno thanks!

I remember using a bench vise to prebleed the MC...is that still true?

[datzenmike]

Do it on the vehicle using the brake pedal.

 

On the fender side of the master is a front and rear bleeder nipple. Run a hose into a container to catch the fluid so it doesn't remove the paint, make sure the reservoirs are full and don't let them empty. Loosen the bleeder, have someone depress the pedal and hold, close the bleeder, let the pedal up. Repeat till no bubbles and only fluid comes out. Refill the reservoirs. 

[abbylind]

Thank you!

[abbylind]

Finally got back to my 620, and installed a new MC. Filled the reservoirs with DOT3 and let it sit for a bit. Without bleeding anything I got a pretty solid pedal! When I can drum up some help I'll bleed the entire system. Thanks everyone for the advise!

[datzenmike]

Don't forget the NLSV under passenger seat

[abbylind]

Copy...thx