That's what the proportioning valve is for, besides balancing the brakes front to rear ....it also makes the opposite end work when the lines blow ...although "work" is a relative term. They'll work ... just NOT very well.
I remember back in my ultra-poverty days, driving my bent '85 Monte up 146 from Worcester. I was in my early 20's, so I was right on the guy in front of me even though he was doing 20 over the 45 limit at the time. Then everyone locks 'em up .... and I slam them on, they grab for a second, then go straight to the floor. I glanced right just before I would've plowed this guy and was able to cut someone off and avoid a ass-end collision, which in MA is 100% YOUR fault (as it should be!). So I drove the car like that for 2 weeks until I got the $12 for a new front hose. The same day I threw the hose on, I was headed back to my house when this shithead in front of me decided to slam on his brakes and take a right turn with NO directional. Slammed my brakes on, BAM! To the floor again - the OTHER side blew this time!
Swerved and missed his bumper by a half inch. I hopped out of the car like a typical 20 year old and started hurling obscenities at the top of my lungs. They didn't stop.
Drove for another 2 weeks like that. I became an expert at driving with no brakes. Probably put 1,000 miles on that thing with just the rears barely working. Never replaced the shoes or anything ....