How to specify break points in legend for geom

I'm making a sort-of heat map using geom_tile, and I'm running into a small aesthetic issue with the legend. As is, I'm generating breaks in the legend using the number of levels in my data, and the plot looks great, but the breaks aren't equally spaced, and are non-integer valued.

Does anyone know how to write the breaks argument such that breaks in the legend are equally spaced and integer valued?

Here's my code:

# Fake data
Freq <- rep(1:13, 15)
SD <- c(rep(1, 13), rep(2, 13), rep(3, 13), rep(4, 13), rep(5, 13), rep(6, 13), rep(7, 13), rep(8, 13), rep(9, 13), rep(10, 13), rep(11, 13), rep(12, 13), rep(13, 13), rep(14, 13), rep(15, 13))
Intro_0 <- sample(80:120,195,TRUE)
test2 <- cbind(Freq, SD, Intro_0)

# Setting up colours
colours <- colorRampPalette(c("deeppink4", "violetred", "palevioletred", "rosybrown1"))(165)
test2$Intro_0 <- factor(test2$Intro_0)
N <- nlevels(test2$Intro_0)

# Plotting
ggplot(test2, aes(x = Freq, y = SD, fill = Intro_0)) 
  geom_tile() 
  xlab("Frequency of Disturbance")  
  ylab("Magnitude of Disturbance") 
  labs(fill="Longevity") 
  scale_x_discrete(labels= c("Once every 10 days", " ", " ", "Once per week", " ", "Once every 5 days", " ", " ", "Once every 2 days", "Once per day", "Twice per day", " ", "10 times per day")) 
  scale_fill_manual(values=colours, breaks=levels(test2$Intro_0)[seq(1, N, by=30)]) 
  #scale_fill_manual(values=colours, breaks=c(80, 90, 100, 110, 120)) 
  theme_tufte() 
  theme(axis.text.x = element_text(angle = 45, hjust=1))

I've tried this:

scale_fill_manual(values=colours, breaks=c(80, 90, 100, 110, 120))

But the legend just disappeared?

Thanks so much for any help!