Brake Master Cylinder and Booster Troubleshooting (2 Viewers)

[MatthewMcD]

I recently replaced my rear drum brakes on my '75 FJ40. The front has disks and the PO recently had new shoes fitted. After flushing the whole system I kept it on the jack stands and ran the engine in 2WD. The wheels turn, but the brake pedal goes straight to the floor. Looking at the master cylinder reservoirs the front is clean the rear is very dirty...

Based on this I assume I need a new master cylinder.

How do I know if I need a new booster?

 

[charliemeyer007]

Leaking booster sound like a vacuum leak - it will cause cylinder 5 to run lean and burn up. Pinch the line with a clamp or hemostats while the engine is running - if it changes rpm's a lot that's a sign the booster is leaking.

You have a front and rear brake system. The master cylinder can fail inside and cause your issue. If the master is good and the disks are good the pedal will not bottom out normally.

Bleed the entire system and look for leaks while you are doing that, brake fluid is cheap. Then adjust the rear drum brake shoes. Now you can see what your brake pedal does - if it goes to the floor the master is bad.

I use a mighty vac aka one man brake bleeder. If you replace the master, bench bleeding it first is helpful. Tons of good youtube vids.

 

[MatthewMcD]

Leaking booster sound like a vacuum leak - it will cause cylinder 5 to run lean and burn up. Pinch the line with a clamp or hemostats while the engine is running - if it changes rpm's a lot that's a sign the booster is leaking.

You have a front and rear brake system. The master cylinder can fail inside and cause your issue. If the master is good and the disks are good the pedal will not bottom out normally.

Bleed the entire system and look for leaks while you are doing that, brake fluid is cheap. Then adjust the rear drum brake shoes. Now you can see what your brake pedal does - if it goes to the floor the master is bad.

I use a mighty vac aka one man brake bleeder. If you replace the master, bench bleeding it first is helpful. Tons of good youtube vids.

Thanks, after bleeding the brakes I don't have any leaks but the rear reservoir turned dark brown while the front stayed clean. So I am pretty certain the master is corroded internally.

I see that Cruiser Corp lists their model as "for drum brakes" but it's a match to my model: Brake Master Cylinder FJ40, FJ45, FJ55 1970-1975 - https://cruisercorps.com/collections/brake-master-cylinder/products/brake-master-cylinder-1 though out of stock. SOR has one for a stupid high price.

Are after market master cylinders from Centric and Dorman reasonable substitutes?

 

[charliemeyer007]

My 72 has 4 drums, I know later they went to front disks. It pays to make sure you order the correct part. Up at the top there is a list vendors, email is cheap shop around.

Plus I'm like the 8th owner and I know what #7 did because he was friend - the other PO's no clue. I got my 72 in 82 or 83. and it sat rusting from like 2010 until a few months ago. Hard to remember all the stuff I did, should have wrote it all down.